Stat Juking?

Labour claims that National's instructed the police to charge fewer people to meet crime reduction targets.  HT: NoRightTurn

“Front line police and others in the criminal justice system are telling us police have had pressure put on by senior officers to reduce the number of charges they lay to meet the Government's targets,” Justice spokesperson Andrew Little says.

“Police are increasingly using pre-charge warnings as a device to not proceed with charges. At the same time I have heard of people being told to gather evidence themselves before police will consider bringing charges.

Labour’s Police spokesperson Jacinda Ardern said New Zealanders were owed an explanation.

“We won’t stop the cycle of repeat offending against women and children by lowering the threshold for prosecution.

“This directive coincides with a significant drop in the number of family violence prosecutions, while at the same time the number of family violence investigations has soared.

“It doesn’t help that police are still not recording domestic violence offences separately, or that access to many programmes aimed at stemming family violence are contingent on a prosecution.”

If it were true, how could we tell?

First off, the greater use of pre-charge warnings isn't a secret. It was something advertised as a deliberate move to free up police resources rather than bring charges for minor offences. Here's the Herald from 2012; here's 3 News from 2010.

The Police's fact sheet on Pre-Charge Warnings notes that they're only used for relatively minor offences:

Operationally, PCWs are more suited in urban rather than rural areas, and in particular the centres of larger localities with prevalent disorder or alcohol-related offending. The initiative targets those 17-30 years of age (highest rates of overall offending are in this age group, including for the key offences eligible for PCWs). Of the top five offences where PCWs are most commonly used, four have no victims (and are “Police initiated”, such as Possession of Cannabis or Disorderly Behaviour). Shoplifting under $500 is the only offence often resolved with a PCW with a recorded victim. 

That doesn't mean that there isn't juking, just that there mightn't be a presumption of juking. What would juking look like?

• There has to be some optimal use of PCWs rather than taking offenders through the courts: low-level offences where a scare should be enough. If the rate of subsequent offending among PCW offenders were higher than the rates among those formally charged, this could be suggestive of too many offenders going through PCW, and especially if the re-offending rates were increasing with increased use of PCWs without subsequent ratcheting back of PCWs by police.
• For non-PCW areas like family violence, we'd expect to see an increasing divergence between crime rates as measured by charged offences and crime rates as measured by survey responses to questions like "Have you been a victim of crime in the last six months". 
• There are crime victimisation questions in the NZ GSS, but it's only updated every two years. 
• The Ministry of Justice maintains the NZ Crime and Safety Survey, but the last iteration of it was 2009. 
• It isn't survey data, but if the hospitals maintain data on the source of ED-presented injuries, you could look for a growing divergence in the number of assaults backed out of that kind of source and the police charge rate. 
• In the absence of frequent survey data on crime victimisation rates, you might look to see whether policing districts with higher ex ante crime rates had increased conviction rates with declining charge rates. If the police were under pressure to reduce the number of charged offenders to keep the number of offences down, you'd hope they'd at least decide to avoid pursuing the cases that were least likely to yield convictions. If these pressures were then different across policing districts because of different crime rates, I'd expect that:
• A greater proportion of charges in juked districts fall on repeat offenders rather than first-time offenders;
• A greater proportion of charges in juked districts proceed to conviction as fewer of the less-certain cases get pursued.
• I'd expect some action in the time path, like districts getting close to some target crime rate start slowing their charge rate more quickly than we'd expect from mean reversion.

I'd be pretty surprised if the police here were juking the stats: that Little's presenting the use of PCWs as evidence of juking, when it was rather well announced policy, doesn't give me confidence. I've not looked at all at the kinds of statistics suggested above. But it's what I'd expect somebody laying accusations of stat-juking to be presenting.

UPDATE: Farrar notes that it shouldn't even be possible to juke the stats by failing to charge as the main crime stat series is based on recorded complaints. I'd assumed that Labour was effectively alleging that the police were juking things by failing to record complaints which didn't proceed to charge, which was part of my "I'd be pretty surprised" prior.