5.4 - Examples

Case- Control Study Example Section

Source: Obesity is associated with decreased risk of microscopic colitis in women



Microscopic colitis is a leading cause of diarrhea in the older adults. There is limited information about risk factors. We hypothesized that obesity would be associated with microscopic colitis.


To examine the association between obesity and microscopic colitis in men and women undergoing colonoscopy.


We conducted a case-control study at the University of North Carolina Hospitals. We identified and enrolled men and women referred for elective, outpatient colonoscopy for chronic diarrhea. We excluded patients with a past diagnosis of Crohn’s disease or ulcerative colitis. A research pathologist reviewed biopsies on every patient and classified them as microscopic colitis cases or non-microscopic colitis controls. Patients provided information on body weight, height, and exposure to medications via structured interviews or Internet-based forms. The analysis included 110 patients with microscopic colitis (cases) and 252 non-microscopic colitis controls. Multivariable analyses were performed using logistic regression to estimate odds ratios and 95% confidence intervals.


Cases were older and more likely than controls to be white race. Study subjects were well educated, but cases were better educated than controls. Cases with microscopic colitis had lower body mass index than controls and reported more weight loss after the onset of diarrhea. Compared to patients who were normal or under-weight, obese (BMI > 30 kg/m2) patients were substantially less likely to have microscopic colitis after adjusting for age and education, adjusted OR (aOR) 0.35, 95% confidence interval (CI) 0.18-0.66). When stratified by sex, the association was limited to obese women, aOR 0.21, 95%CI: 0.10-0.45. Patients with microscopic colitis were more likely to report weight loss after the onset of diarrhea. After stratifying by weight loss, there remained a strong inverse association between obesity and microscopic colitis, aOR 0.33, 95%CI: 0.10 – 1.11 among the patients who did not lose weight. Ever use of birth control pills was associated with lower risk of microscopic colitis after adjusting for age, education, and BMI, aOR 0.38, 95%CI: 0.17-0.84.


Compared to controls also seen for diarrhea, microscopic colitis cases were less likely to be obese. Mechanisms are unknown but could involve hormonal effects of obesity or the gut microbiome.

Case-Crossover Study Example Section

Source: Valent F, Brusaferro S, Barbone F. A case-crossover study of sleep and childhood injury. Pediatrics 2001;107; E23. in Woodward M. Epidemiology: Study Design and Data Analysis. 2nd Ed. London: Chapman and Hall. 2005.

In this Italian case-crossover study of sleep disturbance and injury amongst children (Valent et al., 2001), each child was asked about her or his sleep in the 24 hours before the injury occurred (the case window) and in the 24 hours before that (the control window). Amongst 181 boys, 40 had less than 10 hours sleep on both the days concerned; 111 had less than 10 hours sleep on neither day; 21 had less than 10 hours sleep only on the day before the injury; and 9 had less than 10 hours sleep only on the penultimate day before the injury. The odds ratio (95% confidence interval) for injury, comparing days without and with 10 hours or more sleep, is 2.33 (95% confidence interval; 1.02, 5.79).