Can nonresponse bias and known methodological differences explain the large discrepancies in the reported prevalence rate of violence found in Swedish studies?

INTRODUCTION: The reported prevalence rate of violence varies considerably between studies, even when conducted in similar populations. The reasons for this are largely unknown. This article considers the effects of nonresponse bias on the reported prevalence rate of interpersonal violence. We also single out violence perpetrated in intimate relationships and compare our results to previous Swedish studies. The aim was to explore the reasons for the large discrepancies in the prevalence rates found between studies. MATERIAL AND METHOD: This is a cross sectional study of a random population sample. The NorVold Abuse Questionnaire (NorAQ), covering emotional, physical, and sexual violence, was answered by 754 men (response rate 35%) and 749 women (response rate 38%). Nonresponse bias was investigated in six ways, e.g., findings were replicated in two samples and we explored non-responders’ reasons for declining participation. Also, the prevalence rate of intimate partner violence was compared to four previous studies conducted in Sweden, considering the methodological differences. RESULTS AND DISCUSSION: The only evidence of nonresponse bias found was for differences between the sample and the background population concerning the sociodemographic characteristics. However, the magnitude of that effect is bleak in comparison with the large discrepancies found in the prevalence rates between studies concerning intimate partner violence, e.g., emotional violence women: 11–41% and men: 4–37%; sexual and/or physical violence women: 12–27% and men: 2–21%. Some of the reasons behind these differences were obvious and pertained to differences in the definition and operationalization of violence. However, a considerable proportion of the difference could not easily be accounted for. CONCLUSION: It is not reasonable that so little is known about the large discrepancies in the prevalence rate for what is supposedly the same concept, i.e., intimate partner violence. This study is a call for more empirical research on methods to investigate violence.