Selection bias in pharmacy-based patient surveys


Correspondence to: P. Frisk, Department of Pharmacy, Uppsala University, P.O. Box 580, SE-751 23 Uppsala, Sweden. E-mail:



To evaluate if there is a selection bias in drug utilization surveys on prescription drugs regularly conducted in Swedish pharmacies, to describe the direction of this potential bias and discuss the implications for the results.


Age and gender distributions within patient survey samples from drug utilization surveys conducted during 2006–2010 are compared to the age and gender distribution of all Swedish patients, receiving the same drug or drugs, as given by the Swedish Prescribed Drug Register. The differences between the proportions of patients within the age and gender segments of each pair of survey/register data were calculated.


In 25 (81%) out of 31 included surveys, patients aged 75 years or older are significantly underrepresented, as they are less likely to visit the pharmacy to collect their prescription drugs themselves and thus disqualify for the interviews. Data on women show similar results as overall survey data, whereas the underrepresentation of the oldest age group among men appears in a lower proportion of the surveys, 67%. The general consequence is a selection towards a healthier survey sample, but the consequences in the individual surveys vary, depending on what drug is being studied.


Pharmacy-based patient surveys provide a convenient data collection method for patient self-reported data, but patients aged 75 years or older are consistently underrepresented. In surveys where this may influence the main research question, data should also be collected with other methods reaching the oldest patients. Copyright © 2013 John Wiley & Sons, Ltd.