40% of cops are domestic abusers

It is not rare for anti police violence activist to cite the statistic stating: 40% of all cops are domestic abusers. This statistic is fairly misleading, it comes from an one of study, where only two departments were examined, the study is also 30 years old and for both of those reasons these results cannot be generalized to all of today's cops. 
Often times people call cops "wife beaters" based from this, but this statistic counted the percentage of cops who have done something physically aggressive at least one time, it is wrong to assume that all of these people were abusive on the regular.
Despite all of this, the statistic isn't worthless and there is a conversation to be had about rates of violence among police officers, but activists should be careful about accuracy since this issue is highly political.
== Details ==
The study's methodology is good. The questionnaires were built upon other investigations, there was a pretest, they asked both partners (police officers and their spouses), etc. The content of the questionnaire is not presented (if I am not misremembering), therefore it cannot be evaluated. Per Johnson's description, they asked spouses whether they (and/or their children) had been physically abused at least once, whereas officers were asked whether they had ever gotten out of control and behavior violently. This has implications for interpretation and comparison with other results, but it does not mean the results are meaningless (as oft suggested by those seeking to dismiss the study).
That said, I would argue that it is too liberally cited without proper critical thinking. It is a relatively old report (the study was conducted in the 80s) and it was not conducted in a manner to be generalizable to either "all cops" or "all American cops". They surveyed two "moderate-to-large" East coast departments. We cannot extrapolate from this study that currently "40% of all cops are domestic abusers" unless we assume that the police officers from those two departments are representative of all police officers and that trends in officer-involved domestic violence have remained invariant for decades.
According to Mennicke and Ropes's 2016 review of the literature, the available studies using similar methods to assess rates of domestic violence "operationalized as the self-reported use of physical/domestic violence by a law enforcement officer toward an intimate partner" provide rates from 4.8% to 40%. One of the more recent studies with a large sample size (800+) was conducted by Oehme et al. (2012): the prevalence rate for Floria officers was 28.6%. (Again, it is a convenience sample which cannot be freely generalized, as the authors acknowledge.)