StATS: Examples of confidence intervals (created 2005-06-28).

The following abstracts, all from open source journals, provide good teaching examples of how confidence intervals are used in research publications.

Effects of carrying a pregnancy and of method of delivery on urinary incontinence: A prospective cohort study. Eason E., Labrecque M., Marcoux S., Mondor M. BMC Pregnancy and Childbirth 2004, 4:4 (19 February 2004) Background: This study was carried out to identify risk factors associated with urinary incontinence in women three months after giving birth. Methods: Urinary incontinence before and during pregnancy was assessed at study enrollment early in the third trimester. Incontinence was re-assessed three months postpartum. Logistic regression analysis was used to assess the role of maternal and obstetric factors in causing postpartum urinary incontinence. This prospective cohort study in 949 pregnant women in Quebec, Canada was nested within a randomised controlled trial of prenatal perineal massage. Results Postpartum urinary incontinence was increased with pre-pregnancy incontinence (adjusted odds ratio [adj0R] 6.44, 95% CI 4.15, 9.98), incontinence beginning during pregnancy (adjOR 1.93, 95% CI 1.32, 2.83), and higher pre-pregnancy body mass index (adjOR 1.07/unit of BMI, 95% CI 1.03,1.11). Caesarean section was highly protective (adjOR 0.27, 95% CI 0.14, 0.50). While there was a trend towards increasing incontinence with forceps delivery (adjOR 1.73, 95% CI 0.96, 3.13) this was not statistically significant. The weight of the baby, episiotomy, the length of the second stage of labour, and epidural analgesia were not predictive of urinary incontinence. Nor was prenatal perineal massage, the randomised controlled trial intervention. When the analysis was limited to women having their first vaginal birth, the same risk factors were important, with similar adjusted odds ratios. Conclusions: Urinary incontinence during pregnancy is extremely common, affecting over half of pregnant women. Urinary incontinence beginning during pregnancy roughly doubles the likelihood of urinary incontinence at 3 months postpartum, regardless whether delivery is vaginal or by Caesarean section.

Breastfeeding practices in a cohort of inner-city women: The role of contraindications. England L., Brenner R., Bhaskar B., Simons-Morton B., Das A., Revenis M., Mehta N., Clemens J. BMC Public Health 2003, 3:28 (20 August 2003) Background: Little is known about the role of breastfeeding contraindications in breastfeeding practices. Our objectives were to: 1) identify predictors of breastfeeding initiation and duration among a cohort of predominately low-income, inner-city women, and 2) evaluate the contribution of breastfeeding contraindications to breastfeeding practices. Methods: Mother-infant dyads were systematically selected from three District of Columbia hospitals between 1995 and 1996. Breastfeeding contraindications and potential predictors of breastfeeding practices were identified through medical record reviews and interviews conducted after delivery (baseline). Interviews were conducted at 3–7 months postpartum and again at 7–12 months postpartum to determine breastfeeding initiation rates and duration. Multivariable logistic regression analysis was used to identify baseline factors associated with initiation of breastfeeding. Cox proportional hazards models were generated to identify baseline factors associated with duration of breastfeeding. Results: Of 393 study participants, 201 (51%) initiated breastfeeding. A total of 61 women (16%) had at lease one documented contraindication to breastfeeding; 94% of these had a history of HIV infection and/or cocaine use. Of the 332 women with no documented contraindications, 58% initiated breastfeeding, vs. 13% of women with a contraindication. In adjusted analysis, factors most strongly associated with breastfeeding initiation were presence of a contraindication (adjusted odds ratio [AOR], 0.19; 95% confidence interval [CI], 0.08–0.47), and mother foreign-born (AOR, 4.90; 95% CI, 2.38–10.10). Twenty-five percent of study participants who did not initiate breastfeeding cited concern about passing dangerous things to their infants through breast milk. Factors associated with discontinuation of breastfeeding (all protective) included mother foreign-born (hazard ratio [HR], 0.55; 95% CI 0.39–0.77) increasing maternal age (HR for 5-year increments, 0.80; 95% CI, 0.69–0.92), and infant birth weight ≥ 2500 grams (HR, 0.45; 95% CI, 0.26–0.80). Conclusions: Breastfeeding initiation rates and duration were suboptimal in this inner-city population. Many women who did not breastfeed had contraindications and/or were concerned about passing dangerous things to their infants through breast milk. It is important to consider the prevalence of contraindications to breastfeeding when evaluating breastfeeding practices in high-risk communities.

Randomised controlled trial of a theoretically grounded tailored intervention to diffuse evidence-based public health practice [ISRCTN23257060]. Forsetlund L., Bradley P., Forsen L., Nordheim L., Jamtvedt G., Bjψrndal A. BMC Medical Education 2003, 3:2 (13 March 2003) Background: Previous studies have shown that Norwegian public health physicians do not systematically and explicitly use scientific evidence in their practice. They work in an environment that does not encourage the integration of this information in decision-making. In this study we investigate whether a theoretically grounded tailored intervention to diffuse evidence-based public health practice increases the physicians' use of research information. Methods: 148 self-selected public health physicians were randomised to an intervention group (n = 73) and a control group (n = 75). The intervention group received a multifaceted intervention while the control group received a letter declaring that they had access to library services. Baseline assessments before the intervention and post-testing immediately at the end of a 1.5-year intervention period were conducted. The intervention was theoretically based and consisted of a workshop in evidence-based public health, a newsletter, access to a specially designed information service, to relevant databases, and to an electronic discussion list. The main outcome measure was behaviour as measured by the use of research in different documents. Results: The intervention did not demonstrate any evidence of effects on the objective behaviour outcomes. We found, however, a statistical significant difference between the two groups for both knowledge scores: Mean difference of 0.4 (95% CI: 0.2–0.6) in the score for knowledge about EBM-resources and mean difference of 0.2 (95% CI: 0.0–0.3) in the score for conceptual knowledge of importance for critical appraisal. There were no statistical significant differences in attitude-, self-efficacy-, decision-to-adopt- or job-satisfaction scales. There were no significant differences in Cochrane library searching after controlling for baseline values and characteristics. Conclusion: Though demonstrating effect on knowledge, the study failed to provide support for the hypothesis that a theory-based multifaceted intervention targeted at identified barriers will change professional behaviour.

Family structure and risk factors for schizophrenia: Case-sibling study.  Haukka J.K., Suvisaari J., Lonnqvist J. BMC Psychiatry 2004, 4:41 (27 November 2004). Background: Several family structure-related factors, such as birth order, family size, parental age, and age differences to siblings, have been suggested as risk factors for schizophrenia. We examined how family-structure-related variables modified the risk of schizophrenia in Finnish families with at least one child with schizophrenia born from 1950 to 1976. Methods: We used case-sibling design, a variant of the matched case-control design in the analysis. Patients hospitalized for schizophrenia between 1969 and 1996 were identified from the Finnish Hospital Discharge Register, and their families from the Population Register Center. Only families with at least two children (7914 sibships and 21059 individuals) were included in the analysis. Conditional logistic regression with sex, birth cohort, maternal schizophrenia status, and several family-related variables as explanatory variables was used in the case-sibling design. The effect of variables with the same value in each sibship was analyzed using ordinary logistic regression. Results: Having a sibling who was less than five years older (OR 1.46, 95% CI 1.29-1.66), or being the firstborn (first born vs. second born 1.62, 1.87-1.4) predicted an elevated risk, but having siblings who were more than ten years older predicted a lower risk (0.66, 0.56-0.79). Conclusion: Several family-structure-related variables were identified as risk factors for schizophrenia. The underlying causative mechanisms are likely to be variable.

Overweight, obesity, and colorectal cancer screening: Disparity between men and women. Heo, M., Allison, D. B., Fontaine K. R., BMC Public Health 2004, 4:53 (8 November 2004) Background: To estimate the association between body-mass index (BMI: kg/m2) and colorectal cancer (CRC) screening among US adults aged ≥ 50 years. Methods: Population-based data from the 2001 Behavioral Risk Factor Surveillance Survey. Adults (N = 84,284) aged ≥ 50 years were classified by BMI as normal weight (18.5–<25), overweight (25–<30), obesity class I (30–<35), obesity class II (35–<40), and obesity class III (≥ 40). Interval since most recent screening fecal occult blood test (FOBT): (0 = >1 year since last screening vs. 1 = screened within the past year), and screening sigmoidoscopy (SIG): (0 = > 5 years since last screening vs. 1 = within the past 5 years) were the outcomes. Results: Results differed between men and women. After adjusting for age, health insurance, race, and smoking, we found that, compared to normal weight men, men in the overweight (odds ratio [OR] 1.25, 95% CI = 1.05–1.51) and obesity class I (OR = 1.21, 95% CI = 1.03–1.75) categories were more likely to have obtained a screening SIG within the previous 5 years, while women in the obesity class I (OR = 0.86, 95%CI = 0.78–0.94) and II (OR = 0.88, 95%CI = 0.79–0.99) categories were less likely to have obtained a screening SIG compared to normal weight women. BMI was not associated with FOBT. Conclusion: Weight may be a correlate of CRC screening behavior but in a different way between men and women.

A national survey on the patterns of treatment of inflammatory bowel disease in Canada. Hilsden, R. J., Verhoef, M. J., Best, A., Pocobelli, G. BMC Gastroenterology 2003, 3:10 (5 June 2003) Background: There is a general lack of information on the care of inflammatory bowel disease (IBD) in a broad, geographically diverse, non-clinic population. The purposes of this study were: (1) to compare a sample drawn from the membership of a national Crohn's and Colitis Foundation to published clinic-based and population-based IBD samples, (2) to describe current patterns of health care use, and (3) to determine if unexpected variations exist in how and by whom IBD is treated. Methods: Mailed survey of 4,453 members of the Crohn's and Colitis Foundation of Canada. The questionnaire, in members stated language of preference, included items on demographic and disease characteristics, general health behaviors and current and past IBD treatment. Each member received an initial and one reminder mailing. Results: Questionnaires were returned by 1,787, 913, and 128 people with Crohn's disease, ulcerative colitis and indeterminate colitis, respectively. At least one operation had been performed on 1,159 Crohn's disease patients, with risk increasing with duration of disease. Regional variation in surgical rates in ulcerative colitis patients was identified. 6-Mercaptopurine/Azathioprine was used by 24% of patients with Crohn's disease and 12% of patients with ulcerative colitis (95% CI for the difference: 8.9% – 15%). In patients with Crohn's disease, use was not associated with gender, income or region of residence but was associated with age and markers of disease activity. Infliximab was used by 112 respondents (4%), the majority of whom had Crohn's disease. Variations in infliximab use based on region of residence and income were not seen. Sixty-eight percent of respondents indicated that they depended most on a gastroenterologist for their IBD care. There was significant regional variation in this. However, satisfaction with primary physician did not depend on physician type (for example, gastroenterologist versus general practitioner). Conclusion: This study achieved the goal of obtaining a large, geographically diverse sample that is more representative of the general IBD population than a clinic sample would have been. We could find no evidence of significant regional variation in medical treatments due to gender, region of residence or income level. Differences were noted between different age groups, which deserves further attention.

Long-term benzodiazepine use for insomnia in patients over the age of 60: Discordance of patient and physician perceptions. Mah, L., Upshur, R. E. G. BMC Family Practice 2002, 3:9 (8 May 2002) Background: The aim of this study was to determine and compare patients' and physicians' perceptions of benefits and risks of long term benzodiazepine use for insomnia in the elderly. Methods: A cross-sectional study (written survey) was conducted in an academic primary care group practice in Toronto, Canada. The participants were 93 patients over 60 years of age using a benzodiazepine for insomnia and 25 physicians comprising sleep specialists, family physicians, and family medicine residents. The main outcome measure was perception of benefit and risk scores calculated from the mean of responses (on a Likert scale of 1 to 5) to various items on the survey. Results: The mean perception of benefit score was significantly higher in patients than physicians (3.85 vs. 2.84, p < 0.001, 95% CI 0.69, 1.32). The mean perception of risk score was significantly lower in patients than physicians (2.21 vs. 3.63, p < 0.001, 95% CI 1.07, 1.77). Conclusions: There is a significant discordance between older patients and their physicians regarding the perceptions of benefits and risks of using benzodiazepines for insomnia on a long term basis. The challenge is to openly discuss these perceptions in the context of the available evidence to make collaborative and informed decisions.

Effect of prize draw incentive on the response rate to a postal survey of obstetricians and gynaecologists: A randomised controlled trial. [ISRCTN32823119] Moses S. H., Clark T. J. BMC Health Services Research 2004, 4:14 (28 June 2004) Background: Response rates to postal questionnaires are falling and this threatens the external validity of survey findings. We wanted to establish whether the incentive of being entered into a prize draw to win a personal digital assistant (PDA) would increase the response rate for a national survey of consultant obstetricians and gynaecologists. Methods: A randomised controlled trial was conducted. This involved sending a postal questionnaire to all Consultant Obstetricians and Gynaecologists in the United Kingdom. Recipients were randomised to receiving a questionnaire offering a prize draw incentive (on response) or no such incentive. Results: The response rate for recipients offered the prize incentive was 64% (461/716) and 62% (429/694) in the no incentive group (relative rate of response 1.04, 95% CI 0.96 – 1.13) Conclusion: The offer of a prize draw incentive to win a PDA did not significantly increase response rates to a national questionnaire survey of consultant obstetricians and gynaecologists.

Predicting gender differences as latent variables: Summed scores, and individual item responses: A methods case study. Pietrobon R., Taylor M., Guller U., Higgins L. D., Jacobs D. O., Carey T. Health and Quality of Life Outcomes 2004, 2:59 (25 October 2004) Background: Modeling latent variables such as physical disability is challenging since its measurement is performed through proxies. This poses significant methodological challenges. The objective of this article is to present three different methods to predict latent variables based on classical summed scores, individual item responses, and latent variable models. Methods: This is a review of the literature and data analysis using "layers of information". Data was collected from the North Carolina Back Pain Project, using a modified version of the Roland Questionnaire. Results: The three models are compared in relation to their goals and underlying concepts, previous clinical applications, data requirements, statistical theory, and practical applications. Initial linear regression models demonstrated a difference in disability between genders of 1.32 points (95% CI 0.65, 2.00) on a scale from 0–23. Subsequent item analysis found contradictory results across items, with no clear pattern. Finally, IRT models demonstrated three items to present differential item functioning. After these items were removed, the difference between genders was reduced to 0.78 points (95% CI, -0.99, 1.23). These results were shown to be robust with re-sampling methods. Conclusions: Purported differences in the levels of a latent variable should be tested using different models to verify whether these differences are real or simply distorted by model assumptions.

The outcome of extubation failure in a community hospital intensive care unit: A cohort study. Seymour C. W., Martinez A., Christie J.D., Fuchs B. D. Critical Care 2004, 8:R322-R327 (20 July 2004) Introduction: Extubation failure has been associated with poor intensive care unit (ICU) and hospital outcomes in tertiary care medical centers. Given the large proportion of critical care delivered in the community setting, our purpose was to determine the impact of extubation failure on patient outcomes in a community hospital ICU. Methods: A retrospective cohort study was performed using data gathered in a 16-bed medical/surgical ICU in a community hospital. During 30 months, all patients with acute respiratory failure admitted to the ICU were included in the source population if they were mechanically ventilated by endotracheal tube for more than 12 hours. Extubation failure was defined as reinstitution of mechanical ventilation within 72 hours (n = 60), and the control cohort included patients who were successfully extubated at 72 hours (n = 93). Results: The primary outcome was total ICU length of stay after the initial extubation. Secondary outcomes were total hospital length of stay after the initial extubation, ICU mortality, hospital mortality, and total hospital cost. Patient groups were similar in terms of age, sex, and severity of illness, as assessed using admission Acute Physiology and Chronic Health Evaluation II score (P > 0.05). Both ICU (1.0 versus 10 days; P < 0.01) and hospital length of stay (6.0 versus 17 days; P < 0.01) after initial extubation were significantly longer in re-intubated patients. ICU mortality was significantly higher in patients who failed extubation (odds ratio = 12.2, 95% confidence interval [CI] = 1.5–101; P < 0.05), but there was no significant difference in hospital mortality (odds ratio = 2.1, 95% CI = 0.8–5.4; P < 0.15). Total hospital costs (estimated from direct and indirect charges) were significantly increased by a mean of US$33,926 (95% CI = US$22,573–45,280; P < 0.01). Conclusion: Extubation failure in a community hospital is univariately associated with prolonged inpatient care and significantly increased cost. Corroborating data from tertiary care centers, these adverse outcomes highlight the importance of accurate predictors of extubation outcome.

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