Category: Ask Professor Mean

P.Mean >> Category >> Ask Professor Mean (created 2001-11-24).

Get answers to your Statistics questions from Professor Mean. He's not your average professor! Update: June 2007. This page now contains most of the questions in the Ask Professor Mean format, but there may still be a few stray questions missing. I want to split this page into a set of smaller and more manageable pages. Articles are arranged alphabetically. You can find outside resources at the bottom of this page. Other Ask Professor Mean questions can be found in the Ask Professor Mean page at the StATS website.

All of the material above this paragraph is licensed under a Creative Commons Attribution 3.0 United States License. This page was written by Steve Simon. Anything below this paragraph represents material from my old website, StATS. Until recently (June 2012), this material was available through Children's Mercy Hospital, but is no longer available there. Although I do not hold clear copyright for this material, I am reproducing it here as a service. See my old website page for more details.

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Stats: Accuracy computations (November 26, 2003). Dear Professor Mean, I've heard a lot about accuracy problems with Microsoft Excel, but I'd like to see an example where this really is a problem.

Stats: All or nothing (August 18, 1999). Dear Professor Mean, I would like to know the minimum number of patients needed in order to achieve statistical significance. I am assuming a perfect research situation where all of the patients who got a treatment lived and all the patients who got the placebo died. What would the proper sample size for an all or nothing response be?-- Hesitant Harrison

Stats: Alternating treatments (August 31, 2000). Dear Professor Mean, I'm running an experiment where I randomize by alternating between the treatment and the control. I was told this is not the proper way to do this. Why not?

Stats: Analyzing data under an Intention to Treat model (December 19, 2007). Dear Professor Mean, I need to know how to analyze a data set using the intention to treat principle.

Stats: Are we assuming a normal sample or a normal population? (August 30, 2007). Dear Professor Mean, I'm fitting an ANOVA model to a sample of 25 observations, and the data is skewed. I'm quite worried about this, but my husband reassures me that this is not a problem. He says that the assumption is that the population is normal, not the sample. Should I listen to him?

Stats: Asymmetric confidence intervals (September 3, 1999). Dear Professor Mean, I found a journal article with a confidence interval that was asymmetric. For example, the authors reported a mortality difference of 5% and a 95% confidence interval of -1.2% to 12%. I can't understand how the CI can be unequally distributed if it uses the form ESTIMATE +/- 1.96*STANDARD ERROR.

Stats: Best fitting curve (January 26, 2000). Dear Professor Mean: I have a graph of the trend for the mean frequency of injuries among children from 1 to 11 years of age. The shape of the curve suggests a nonlinear relationship between the age and the frequency of injuries. Is there some software that would provide the best fitting curve for this data from among a large family of nonlinear curves?

Stats: Binary outcome sample size calculations (August 23, 2000). Dear Professor Mean, I have to calculate a sample size for a binary outcome variable. The research study is on breast feeding failures within 7 to 10 days of birth for mothers who intended to breast feed. The rate of failure overall is expected to be about 12%. What sample size do I need? -- Baffled Bob

Stats: Bonferroni correction (September 3, 1999). Dear Professor Mean: I keep reading about something called a Bonferroni correction. Somehow this method keeps researchers from going on a fishing expedition. Could you explain what a Bonferroni correction is and why we want to keep scientists from fishing? -- Judicious John

Stats: Bootstrap (January 26, 2000). Dear Professor Mean: I've heard a lot about how the bootstrap is going to revolutionize statistics. How does the bootstrap work?

Stats: Calculating statistics on whole numbers (April 4, 2008). Dear Professor Mean, I have some data that has a resolution of 1, meaning that these are whole numbers only and no decimals. When I calculate the mean and standard deviation, should I report the actual value or should I round it?

Stats: Can the p-value actually equal 1.0? (May 30, 2006). Dear Professor Mean, I have a data set that compares the proportions in two groups. In the first group, the proportion is 19% (5/26). In the second group, the proportion is also 19% (3/16). I computed a p-value of 1.0 for this data, but a referee tells me that a p-value of 1.0 is impossible. How can I convince the referee that he/she is wrong.

Stats: Can the standard deviation be more than half of the range? (June 22, 2007). Dear Professor Mean, I was trying to work with some simple data sets to see how large I could make the standard deviation relative to the range. I know the standard deviation can never be larger than the range, but I can't seem to get it to be larger than half the range.

Stats: Can you use the coefficient of determination for categorical variables (April 4, 2006). Dear Professor Mean, How can you compute a coefficient of determination (R squared) for a model that has a dichotomous variable? I thought that you could only compute this in a linear regression model?

Stats: Causation (January 27, 2000). Dear Professor Mean: Everyone says that smoking causes cancer, but we can't really say that, can we? There is an association between smoking and cancer, but we know that association does not imply causation, don't we?

Stats: Central Limit Theorem (March 9, 2004). Dear Professor Mean, How does the central limit theorem affect the statistical tests that I might use for my data?

Stats: Checking the assumption of normality (September 11, 2002). Dear Professor Mean, I have some data that don't seem to meet the assumption of normality. What should I do? -Anxious Abby

Stats: Chi-square (September 3, 1999). Dear Professor Mean: Can the Chi-squared test be used for anything besides categorical data?

Stats: Confidence interval for a rate (October 10, 2007). Dear Professor Mean, How do you calculate a confidence interval for a rate?

Stats: Confidence intervals (November 29, 2004). Dear Professor Mean: Can you give me a simple explanation of what a confidence interval is?

Stats: Collinearity (January 27, 2000). Dear Professor Mean: Could you describe the term collinearity for me? I understand that it has to do with variables which are not totally independent, but that is all I know!

Stats: Composite scores (January 27, 2000). Dear Professor Mean: I have developed a method to distinguish among several products that we need to buy so our company can make a good purchasing decision. I created a composite score which is a weighted average of several different indicators of quality. I want to use statistics to determine when two different products have significantly different composite scores.

Stats: Confidence interval with zero events (January 19, 2001). Dear Professor Mean, I was working with a colleague on some confidence intervals for the probability of an adverse event during several different types of operations. One of the proportions was zero, since the event never occured. My friend computed a confidence interval and it went from zero to zero. I told him that this couldn't be right and computing a confidence interval with zero events is impossible. Isn't that right? -- Killjoy Karlina

Stats: Data is/are (November 26, 2003). Dear Professor Mean, Should I write "data is" or "data are"?

Stats: Date calculations in SPSS (August 18, 1999). Dear Professor Mean, I am trying to use dates in SPSS for certain calculations. For example, I want to use a compute statement in SPSS to create a new variable called duration of injury (durinj). I know that I must subtract the date of injury from the date of interview. However, when I do this, I get a number in the millions. What am I doing wrong? -- Stumped Sharon

Stats: Degrees of Freedom (September 3, 1999). Dear Professor Mean: In your Simple Descriptive Statistics class, you described the standard deviation as the square root of the average squared deviation. If it is an average, how come we divide by the degrees of freedom (n-1) rather than n. Is this just a conspiracy among statisticians to make this stuff harder to understand?

Stats: Designing a pilot study (September 13, 1999). Dear Professor Mean, I am proposing a research study that will examine a complex intervention of diet, exercise, and behavioral modification for some of my pediatric patients who need to lose weight. I want to collect some data from a pilot study before I start the research study. How do I describe the pilot study in my protocol? -- Sophisticated Sarah

Stats: Developing a research hypothesis (August 18, 1999). Dear Professor Mean, I want to do some research, but before the hospital won't approve anything until I have a protocol with a research hypothesis. I'm not sure why a research hypothesis is important. Can you help? -- Little Linda

Stats: Displaying tables of percentages (November 6, 2002). Dear Professor Mean, My colleagues and I argue over the most appropriate way for displaying tables of percentages. Must the row or column always add to 100%? Also, in cases where it is difficult to know which variable is dependent, how does one decide the best way to present the results? -- Garrulous Gail

Stats: Documenting negative results in a research paper (October 11, 2001). Dear Professor Mean, I have just finished a well-designed research study and my results are negative. I'm worried about publication bias; most journals will only accept papers that show positive results. How do I document the negative findings in a research paper in a way that will convince a journal to accept my paper? -- Apprehensive Arturo

Stats: Documenting your SPSS data sets (August 18, 1999). Dear Professor Mean, I need to add some documentation for SPSS data sets that I am creating. I know you covered this in your "Gentle Introduction to SPSS" class, but I've already forgotten everything. Can you review this for me? -- Baffled Bill

Stats: Does prevalence affect sensitivity (January 31, 2008). Dear Professor Mean, Does lowering the prevalence of a disease have an effect on sensitivity?

Stats: E notation (September 3, 1999). Dear Professor Mean: In my regression output, I saw that following value: -2.384E-03. What does this E notation mean?

Stats: Establishing validity and reliability (November 6, 2002). Dear Professor Mean, I need to establish validity and reliability of a new measurement. How do I do this? --- Flustered Fred

Stats: Ethics concerns about a placebo run-in (October 4, 2006). Dear Professor Mean, Some of the trials that our Institutional Review Board looks at have a placebo run-in period. All patients are given a placebo before the start of treatment and anyone who responds well to the placebo is dropped from the trial. What are the ethical ramifications of such a study. You can't disclose the placebo run-in period to the research volunteers because it would defeat the purpose.

Stats: Ethics of a placebo group (August 2, 2001). Dear Professor Mean, Some of my colleagues want to use placebos in their research, but I have warned them about the ethical issues surrounding the use of a placebo group. When (if ever) is it ethical to use a placebo group? --Kibitzing Kathy

Stats: Exploring interactions in a linear regression model (August 1, 2002). Dear Professor Mean, I have a model with two factors. When I ran the model, it showed a significant interaction between the two factors. What do I do now? --Troubled Trudy

Stats: Exporting SPSS graphs and tables (January 28, 2000). Dear Professor Mean, I need to export the output from SPSS and use some of it in my word processing file. What is the best way to do this? -- Manic Marsha

Stats: Fisher's Exact Test (August 23, 2000). Dear Professor Mean: What is Fisher's Exact Test and when should I use it?

Stats: Flaws in a research paper (January 27, 2000). Dear Professor Mean: A small group I've been teaching has gotten extremely interested in how to decide when there are too many flaws in a paper which would completely invalidate (and circular file) it.

Stats: General guide to data entry (September 3, 1999). Dear Professor Mean, I'm about to start typing in my research data. Do you have any general guidelines for data entry?

Stats: Getting IRB approval for your research Dear Professor Mean: I am submitting a proposal to our Institutional Review Board. Is there anything you can do to help me get IRB approval? --Terrified Terri

Stats: Guidelines for ANOVA models (June 20, 2003). Dear Professor Mean, I wanted to compare two groups in my research, those who completed every test battery, and those who completed only some of them. I ran ANOVAs on age, iq, adhd score, and so forth. My professor says that I should have used a t-test instead. Why can't I use ANOVA. Isn't ANOVA better than a t-test? --Angry Anastasia

Stats: Guidelines for poisson regression models (September 21, 1999). Dear Professor Mean, I have just received feedback on a manuscript under review in which one reviewer recommended use of Poisson regression. I am not familiar with this technique--when it is appropriate and/or recommended, what assumptions the data must meet, whether the procedure in SAS? SPSS? I would appreciate a reference and/or citation to article(s) in which it has been used. Thanks! -- Denied Denise

Stats: Guidelines for reliability/validity models (January 13, 2004). Dear Professor Mean, How do I show that a measurement has validity and reliability? Meek Mark

Stats: How good is my prediction? (August 13, 2007). Dear Professor Mean, I have two time series of data, one actual and one predicted. Since I'm quite new to statistical methods, I would like to know what methods are used to evaluate the different between the two time series. I would like to able to say something like "the predicted values were 70% accurate."

Stats: Importing database files into SPSS (August 18, 1999). Dear Professor Mean, How do I import database files into SPSS? I don't want to re-type everything, because there are 70,000 records. The data are stored in a Microsoft Access file. -- Vexed Vidya

Stats: Importing spreadsheet data into SPSS (August 20, 1999). Dear Professor Mean, I need to import data in an Excel spreadsheet, but I can't get SPSS to read this data properly. Can you help? -- Stumped Stan

Stats: Injury index creation (September 23, 1999). Dear Professor Mean: I want to create an injury index that describes the severity of an injury to a child. This would include information about the type of injury, the location of the injury, the age of the child, etc. What's the best way to do this?

Stats: Intention to treat (January 27, 2000). Dear Professor Mean: I'm confused by medical journal articles that talk about the use of "Intention to Treat" analysis. What does this term mean?

Stats: Interim analysis (September 13, 1999). Dear Professor Mean, I'm going on a job interview and I know one of the questions they will ask me is about interim analysis. What should I tell them? -- Harried Howard

Stats: IRB review of a pilot study (March 26, 2007). Dear Professor Mean: I am the new chair of the IRB at a county hospital. Many of the studies we review are pilot studies with small samples. I have been trying to locate criteria for the scientific review of pilot studies, but have not found a consensus in the literature that I have seen. Is a pilot study merely a "dry run" of the procedures that will be used in a later, larger-scale study? Or, is it reasonable for the IRB to demand that the investigator provide specific criteria for determining whether the pilot has been a success? And, should the IRB furthermore demand that specific hypotheses be formulated? My impression is that many investigators declare their studies to be pilots in order to avoid more rigorous scrutiny of their proposals.

Stats: Is my confidence interval too wide? (September 21, 2006). Dear Professor Mean, Is there a rule of the thumb to judge if a 95% CI is wide or narrow?

Stats: Is my confidence interval wide? (September 11, 2007). Dear Professor Mean, I have a case-control design. Among the cases, 271 were exposed and 317 were unexposed. Among the controls, 125 were exposed and 976 were unexposed. After adjustments for covariates, this produced an odds ratio of 7.4 with a 95% confidence interval of 5.7 to 9.5. Is this a wide interval?

Stats: Jargon in Statistics (January 27, 2000). Dear Professor Mean: I have to review a paper for journal club and I don't understand all the obscure statistical jargon that the authors use.

Stats: Kaplan Meier (June 27, 2000). Dear Professor Mean: When I read my medical journals, I keep on coming across terms like "Kaplan-Meier Product Limit Estimate" or "Kaplan-Meier survival curve." What do these terms mean and when are they used?

Stats: Log transformation Dear Professor Mean, I have some data that I need help with analysis. One suggestion is that I use a log transformation. Why would I want to do this? -- Stumped Susan

Stats: Longitudinal data (July 26, 2002). Dear Professor Mean, I have longitudinal data on the growth pattern of patients given growth hormone. How should I store the data? --Jittery Jerry

Stats: Maximum likelihood estimation (May 6, 2003). Dear Professor Mean: What is maximum likelihood estimation and how does it work?

Stats: Mean or median? (July 28, 2003). Dear Professor Mean: I am writing a report on turnover. I want to summarize the number of weeks it takes to fill a vacancy. Should I use a mean or a median?

Stats: Merging files in SPSS (January 15, 2004). Dear Professor Mean, I get a strange error message when I try to merge two files in SPSS. What is going on? -- Computing Cheryl

Stats: Mixture models (January 27, 2000). Dear Professor Mean: I have read a journal article where the authors used a mixture model . What is this?

Stats: Modifying SPSS data (August 18, 1999). Dear Professor Mean, Before I start my data analysis, I need to modify some of the data in my SPSS data set. I don't want to re-type every number by hand. Is there a faster way to do this? -- Impatient Pam

Stats: Number needed to treat (January 27, 2000). Dear Professor Mean: How are patients and their doctors supposed to decide whether a research finding has practical significance? Why don't the medical journals make things clearer?

Stats: Odds ratio versus relative risk (January 9, 2001). Dear Professor Mean: There is some confusion about the use of the odds ratio versus the relative risk. Can you explain the difference between these two numbers?

Stats: One-tailed p-values (April 12, 2004). Dear Professor Mean, SPSS produces two-tailed p-values, but I want a one-tailed p-value. How do I get this?

Stats: The order of entering interactions into a model (September 20, 2007). Dear Professor Mean, I like your titanic example! But shouldn't you enter the interaction term on a second step following entry of the main effects on the first step? If you enter the terms all at the same time, the interaction term will compete for variance with the two main effects on which is depends.

Stats: Outliers (January 28, 2000). Dear Professor Mean: I have recently conducted a survey of attitudes toward research from a professional group. There are some outliers (+/- 3SD) that I would eliminate , but others conducting the research with me feel that this might be a minority view, and should not be eliminate from the dataset......any views or references that I should read to confirm my view, or theirs?

Stats: Page's test (September 3, 1999). Dear Professor Mean: I have recently come across a statistical test (Page's L test), with which I am unfamiliar. Does anyone either have information about this test or know where I might find information about it?

Stats: Parametric tests for a ratio (October 27, 2006). Dear Professor Mean, I computed a variable, Y3, which is the ratio of two other variables, Y1 and Y2. Can I use a parametric test on this ratio?

Stats: Parametric versus nonparametric tests (July 30, 2001). Dear Professor Mean: When should I use a parametric test versus a non-parametric test?

Stats: Physician Performance Data (January 27, 2000). Dear Professor Mean: Producing statistics of physician performance or group performance or whatever seems to be one of the great growth industries in medicine. Graphs of performance in just about anything seem to be produced - usually with something that looks at first glance like a normal distribution (and almost never with any statistical addenda). But I would like to know whether we can use them sensibly as anything other than pictures? In particular when I am one of the subjects of the analysis how do I interpret my own performance?

Stats: Post hoc comparisons (March 15, 2006). Dear Professor Mean, I need to run multiple comparisons among all possible pairs of means following an analysis of variance test. What is the best approach? Tukey? Scheffe? Bonferroni?

Stats: Post hoc power (November 1, 2002). Dear Professor Mean, The results of my study were negative, and the journal reviewer insists that I perform a post hoc power calculation. How do I do this? -Jittery Jerry

Stats: Privacy concerns in research (July 12, 2002). Dear Professor Mean, I want to do some research using tissue samples, but the Institutional Review Board has said that I have to get consent first, because the data are not anonymized. The also told me that I might be able to get a waiver from consent if I deidentify the data. What's up with all these privacy concerns in research. -- Doubting Denise

Stats: Protocol changes (December 22, 2000). Dear Professor Mean: After I collected my data, I noticed a problem that I had not anticipated. I want to make some protocol changes and analyze my data differently. Can I do this?

Stats: Quick sample size calculations (October 11, 2001). Dear Professor Mean, I'm reading a research paper. I suspect that the sample size is way too small. I don't like the findings of the study anyway, so I'm hoping that you will help me discredit this study. Is there a quick sample size calculation that I can use? -- Cynical Chris

Stats: Quota stratified random sampling (January 28, 2000). Dear Professor Mean: I'm doing my thesis right now and I have to use the quota stratified random sampling. I do not know the exact meaning of this term.

Stats: Randomization (August 18, 1999). Dear Professor Mean, I need to randomize the order in which I give treatments and controls in my research study, but I don't know how to randomize. Can you show me what to do? -- Baffled Beth

Stats: Regression to the mean (January 27, 2000). Dear Professor Mean: In a stat course, I was introduced to the term "regression to the mean". Today we administered a pretest to 4th graders. In February we will test again, with the same exam, to see "how much they've learned". I explained to the principal that, of course they would do better, no matter how well they were taught, that this was a classic case of regression to the mean. Am I correct, close, or way off on this?

Stats: Relationship between the standard deviation and the sample size (May 26, 2006). Dear Professor Mean, I have a data set that is accumulating more information over time. When I estimate the standard deviation for one of the outcomes in this data set, shouldn't that value decrease as the sample size increases?

Stats: ROC curve (August 18, 1999). Dear Professor Mean: I was at a meeting in Belgium and the buzz statistic was ROC Analysis. I think it stands for Receiver Operating Characteristic curve. It seems to be used for predictive values. I seemed to be a lone ranger in not understanding as they were showing in several presentations "by this curve you can see this is good or bad" and they didn't look very different. Do you have a simple explanation about ROC curves?

Stats: R-squared (August 18, 1999). Dear Professor Mean: On my TI-83, when calculating quadratic regression, there is a number that is found called R-squared (R^2). I understand that this is the coefficient of determination. But....I thought that R^2 had to do with linear models. What is R^2 finding for this quadratic regression? what does this number mean? is there a way to find R^2 through a "pencil and paper" process? I know the equation for R^2 for a linear regression. But its the quadratic I need to know about. Please, anyone, help!!

Stats: Sample size for a confidence interval (January 26, 2000). Dear Professor Mean, We have a large dataset with about 400 million records. We need to randomly select a subsample from it. However we need help in determining the sample size. What sample size do we need for the confidence interval calculations? -- Frantic Frank

Stats: Sample size for a diagnostic study (September 3, 1999). Dear Professor Mean, How big should a study of a diagnostic test be? I want to estimate a sample size for the sensitivity and specifity of a test. I guess confidence intervals would address this, but is there a calculation analogous to a power analysis that would apply to figure out the size of the groups beforehand? -- Jovial John

Stats: Sample size for Mann-Whitney U (September 28, 2000). Dear Professor Mean, I need to calculate the sample size for the Mann-Whitney U test. How do I do this? -- Bewildered Bob

Stats: Sigma in the control chart (January 27, 2000). Dear Professor Mean: I ran a control chart in SPSS for individual values, and the control limits don't correspond with what I would expect from the descriptive procedure that I ran first. In particular, the value of sigma in the control chart appears to be an approximation of what I computed earlier. Why would SPSS use a different calculation for sigma?

Stats: Skewed data (June 5, 2003). Dear Professor Mean: Please explain how the standard deviation can be greater than the mean. I think it is because of skewed data.

Stats: Small sample size (October 11, 2001). Dear Professor Mean, Are there problems with a very small sample? Can the t-test be used with a sample of just three subjects? -- Anxious Abdelwahab

Stats: So you want to write a questionnaire (July 12, 2002). Dear Professor Mean, I need to write a questionnaire for a research study I am conducting. Can you help me write it? -- Cautious Carmen

Stats: Splines (January 27, 2000). Dear Professor Mean: Can you send me a basic definition of splines?

Stats: Spreadsheet or database (January 28, 2000). Dear Professor Mean, I am not sure whether I should use a database or a spreadsheet to enter my data?

Stats: SPSS citation (March 6, 2002). Dear Professor Mean: I'm writing a research paper. When I talk about the statistical methods, how do I properly cite the use of SPSS software? And just what does SPSS stand for anyway? --Zealous Zohreh.

Stats: Stein's paradox (January 27, 2000). Dear Professor Mean: What is "Stein's Paradox?"

Stats: Stopping a study early (October 29, 2002). Dear Professor Mean, I tried really hard to recruit the number of subjects that I promised to in my power calculations, but I just can't do it. I'm thinking about stopping the study early, but I'm worried that it might screw up all my statistics. -- Exhausted Evelyn

Stats: Stratified random sample (August 23, 2000). Dear Professor Mean: What is a stratified random sample and why would I want to use one?

Stats: The minimal impact of population size on power and precision (January 19, 2001). Dear Professor Mean, Can you explain why it is okay to have similar sample sizes for populations of very different sizes. For example, why is it that a sample size for a population of 10 million doesn't have to be much larger than a sample size for a population of 10 thousand? -- Skeptical Sam

Stats: Three things you need for a power calculation (November 8, 2001). Dear Professor Mean, I want to do research. Is forty subjects enough, or do I need more? Didn't I hear you mention something about three things you need for a power calculation? -- Eager Edward

Stats: T-test (April 18, 1999). Dear Professor Mean: How do you analyze a t-test? I have a t-test value, and I know that I have to compare it to a t-distribution. I'm not sure how to do that.

Stats: Type II error (September 3, 1999). Dear Professor Mean: A journal reviewer criticized the small sample size in my research study and suggested that I mention a Type II error as a possible explanation for my results. I've never heard this term before. What is a Type II error?

Stats: Type III error (January 3, 2008). Dear Professor Mean, What is the definition of a Type III error?

Stats: Unequal group sizes (November 2, 2001). Dear Professor Mean: I am comparing several groups of subjects, but the number of subjects in each group differ quite a bit. How does this affect the assumptions in analysis of variance?

Stats: Web polls (July 30, 2001). Dear Professor Mean: I'm conducting a number of web-based polls on the web and would like to include a margin of error in my results. How do I do this?

Stats: What to measure in a post-marketing surveillance study (May 2, 2007). Dear Professor Mean, I am volunteering as a data analyst in a post-marketing surveillance to assess the safety and efficacy of a drug. I'm not sure what to measure and how to measure it. Can you help me figure out what really needs to be done?

Stats: When does heterogeneity become a concern? (June 5, 2008). Dear Professor Mean, I have an ANOVA model and I am worried about heterogeneity--unequal standard deviations in each group. How should I check for this?

Stats: When should you use a log transformation? (December 28, 2007). Dear Professor Mean, How do I know whether it is appropriate to use a log transformation for my data?

Stats: Why 95% confidence limits (May 6, 2002). Dear Professor Mean:, I've been working with small data sets for some neuroimaging research that have five (5) treatment and five (5) control participants. It is not unusual to have such small samples in this kind of work. My 95% confidence interval (CI) included zero; yet, the 85% confidence interval did not include zero. I know that the 95% CI is the common one, but I also know that others are used, but I don't know when to use them. Therefore, I'd like to know why we use 95% confidence limits all the time? When is it appropriate to use other CIs and the logic behind making such decisions?

Stats: Why the plus one in the percentile formula p(n+1)? (June 22, 2007). Dear Professor Mean, I was reviewing your page on the interquartile range and was wondering why the formula for the quartiles in particular and percentiles in general asks you to select the p(n+1) observation. Why do you need to add one?

Stats: Writing a methods section (January 15, 2001). Dear Professor Mean: I am submitting a proposal to our Institutional Review Board. Is there anything you can do to help me get IRB approval? --Terrified Terri

Stats: Writing a research grant (September 13, 1999). Dear Professor Mean, I'm writing a research grant to look at the impact of managed care on the care of children with chronic epilepsy. How do I structure the grant so I'm guaranteed to get funding? -- Ambitious Ann

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This work is licensed under a Creative Commons Attribution 3.0 United States License. This page was written by Steve Simon and was last modified on 2017-06-15.