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To the Editor Mundt et al1 should be applauded for attempting to bring new data to bear on a research question of substantive policy significance. However, we are concerned that their analysis overlooked a point of critical significance. While they argued that their findings “cannot be explained by a simple overall trend to reduce numbers of beds and increase prison populations,” they failed to adjust for secular changes in the underlying determinants of crime.2 Neither did Lamb,3 who argued in the accompanying editorial that longitudinal data are needed to appropriately test the Penrose hypothesis, comment on the need to do so. Such adjustment would be considered standard practice in the analysis of panel data,4 particularly given the challenges of drawing causal inferences from cross-national panel data.5
Tsai AC, Venkataramani AS. A More Robust Test of the Penrose Hypothesis. JAMA Psychiatry. 2015;72(7):735-736. doi:10.1001/jamapsychiatry.2015.0212