StATS: What is a cross sectional design?

A research design where subjects are assessed at a single time in their lives. A cross sectional study is fast and can study a large number of patients at little cost or effort. Also, you don't have to worry about patients dropping out during the course of the study. This study is efficient at identifying association, but may have trouble deciding cause and effect. With data at only one time point, you don't know whether the chicken or the egg came first. Here are two examples of cross sectional designs.

• In Zureik et al (BMJ 2002 Aug 24;325(7361):411), a group of 1132 adults with asthma were given respiratory function tests to assess the severity of their asthma. They were also given skin prick tests to assess their sensitization to mold, pollen, dust mites, and cats. In this study, those patients with reactions to mold were much more likely to have severe asthma.
• In Garrett et al (BMJ 2002 Jun 22;324(7352):1494), a group of 190 mothers of normal healthy infants attending one of nine child care centers in Ireland, were surveyed. These mothers were asked to recall when their babies achieved certain developmental milestones, and whether they used babywalkers. In this study, babies who used baby walkers had significant delays in when they first crawled, stood up alone, and walked alone.

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