StATS: More on searching the literature (May 17, 2005)

I was trying to track down an article that I remembered from several years ago. It was an evaluation of smoking cessation programs for pregnant mothers to try to improve the birth outcomes, especially an increase in birthweight. This query could easily be fit into the PICO format.

Does the use of smoking cessation programs (I) in women who are pregnant (P) lead to an improvement in birthweight (O) compared to simply offering advice and encouragement about the importance of quitting smoking (C)?

Normally, it is best to go to high level sources, such as the Cochrane database,, or first. There is a very nice Cochrane review, which we will see details about later. The search at was a bust, but had a nice guideline

This guideline had the following comments about smoking during pregnancy:

I-5 Screening for Tobacco Use - Offer Cessation -- Week: 6-8

The Working Groupís Recommendations for Women In Low Risk Pregnancy:

  1. Strongly recommend routine screening for tobacco use in pregnancy at the initial prenatal visit. For patients who smoke, recommend assessment of smoking status at each subsequent prenatal visit. (Lumley, Oliver, & Waters, 2001; Mullen et al., 1991) (QE: I; Overall Quality: Good; R: A)
  2. If the screening is positive, cessation should be strongly recommended. (Wisborg et al., 2000; Panjari et al., 1999; Dolan-Mullen, Ramirez, & Groff, 1994) (QE: I; Overall Quality: Good; R: A)
  3. There is insufficient data to recommend for or against pharmacologic therapy for tobacco cessation in pregnancy.

Notice the cryptic notes in parentheses after items 1 and 2. This a grading system on the quality of the evidence, and you have to be sure you understand the codes properly. Some systems use number codes and some use letter codes. Even worse, for some systems, low numbers or letters represent the best quality evidence and for other systems low numbers or letters represent the lowest quality evidence. So you have to read the fine print.

For this particular guideline, QE: I  represents "Evidence obtained from at least one properly randomized controlled trial." You can also discover that "Overall Quality: Good" means that the evidence is directly linked to the health outcome, as opposed to an intermediate or surrogate outcome. Finally, the phrase "R: A" means "A strong recommendation that the intervention is always indicated and acceptable."

For a complex search in PubMed, it can sometimes help to break the search into individual pieces and then combine the pieces together. So you should  first took a look at how many references you would find if you looked at "smoking cessation" (10,621 references), and then pregnancy (570,261 references), and then birthweight (29,446 references). All of these numbers are large, which reassures you that you are searching using the right words. In contrast, if you had searched on "stopping smoking" rather than "smoking cessation," you would have only found 661 references, which is too small a base when you start combining it with the other terms.

Now combine the terms together.

The combination of all of these terms produces 69 references. When you still have a large number of references, you then have the luxury of looking for a meta-analysis of these studies to save yourself the effort of reading a bunch of individual studies. You can click on the LIMITS tab in the PubMed search, or you can add the term "Meta-Analysis [pt]" to your search criteria. This final search yields two meta-analyses

  1. Lumley J, Oliver SS, Chamberlain C, Oakley L. Interventions for promoting smoking cessation during pregnancy. Cochrane Database Syst Rev. 2004 Oct 18;(4):CD001055. Review. PMID: 15495004 [PubMed - indexed for MEDLINE]
  2. DiFranza JR, Lew RA. Effect of maternal cigarette smoking on pregnancy complications and sudden infant death syndrome. J Fam Pract. 1995 Apr;40(4):385-94. PMID: 7699353 [PubMed - indexed for MEDLINE]

 and the first one listed is the one I wanted.

You could have refined this search using MeSH terms. MeSH terms are especially helpful for a term like "heart attack" that is too vague from a medical viewpoint. This vagueness causes two problems. First, "heart" and "attack" are common words with multiple meanings and uses. These words may appear far separated from each other in an article that is totally unrelated to heart attacks. Second, the words "heart attack" could describe conditions like "Myocardial Infarction" or "Coronary Arteriosclerosis" or "Coronary Thrombosis" or "Congestive Heart Failure" and would thus produce too broad a range of conditions.

Some other considerations to improve your search include:

The search strategy I describe here works well on other systems besides PubMed.

This page was written by Steve Simon while working at Children's Mercy Hospital. Although I do not hold the copyright for this material, I am reproducing it here as a service, as it is no longer available on the Children's Mercy Hospital website. Need more information? I have a page with general help resources. You can also browse for pages similar to this one at Category: Information searching.