Were there false reports in the SPLC's post-election hate crime data?

On 29 November, The Southern Poverty Law Centre (SPLC) reported that there had been 867 hate crimes in the United States since the election result was announced. The SPLC's report provided a break down of these 867 hate crimes by state. The authors of the reported noted the following:

The 867 hate incidents described here come from two sources — submissions to the #ReportHate page on the SPLC website and media accounts. Incidents were limited to real-world events; the count does not include instances of online harassment. We have excluded incidents that authorities have determined to be hoaxes; however, it was not possible to confirm the veracity of all reports.

Yesterday on Twitter I came across a plot showing a relatively strong negative relationship between SPLC hate crimes per capita and Trump vote share across US states. (Unfortunately, I can't seem to find the link again, so apologies to whomever made the plot.) In other words, there were more reported hate crimes in states with fewer Trump voters. This suggests that the SPLC might not have been able to exclude all hoaxes or false reports from their data. 

At the suggestion of my friend Roberto Cerina, I investigated this possibility further by comparing the SPLC data to state-level hate crime data from the FBI's Uniform Crime Reports. Specifically, I obtained hate crimes per capita in each state for each year from 2011 to 2015 (as well as Hillary vote share in 2016 for each state). Washington DC was excluded, due to being an outlier. No FBI data were available for Hawaii. 

The correlations between the FBI measures of hate crimes per capita in different years were all very strong, ranging from r = . 78 to r = .92 (p < 0.001 in all cases). This suggests that there is a high degree of persistence in hate crimes per capita from year to year. States with more hate crimes per capita in one year tend to have more hate crimes per capita in the next year. However, the correlations between the FBI measures and the SPLC measure of hate crimes per capita were much weaker, ranging from r = .17 to r = .38 (some significant, some not). Given the high degree of persistence from year to year in the FBI data, I computed the average FBI hate crimes per capita from 2011 to 2015. This variable was correlated with SPLC hate crimes per capita at r = .28 (p = 0.049).

To investigate the possibility of false reporting, I first looked to see whether Hillary vote share or average FBI hate crimes per capita is a better predictor of SPLC hate crimes per capita. Results are shown below:

Consistent with the plot I saw on Twitter, Hillary vote share is strongly associated with SPLC hate crimes per capita. When average FBI hate crimes per capita is added to the model, it enters non-significantly, and Hillary vote share remains a strong predictor. Next, I looked to see whether Hillary vote share is correlated with hate crimes per capita in previous years, as reported by the FBI. Results are shown below:

Hillary vote share is not associated with hate crimes per capita in 2015, 2014, 2013 or 2012. By contrast, the previous year's hate crimes per capita is strongly associated with hate crimes per capita in each year. (It should be noted that there was a marginally significant correlation between Hillary vote share and hate crimes per capita in 2011: r = .26, p = 0.068). In other words, SPLC hate crimes per capita is better predicted by Hillary vote share than FBI hate crimes per capita, whereas FBI hate crimes per capita in any given year is better predicted by the previous year's (or the average) hate crimes per capita.

As an alternative strategy, I regressed SPLC hate crimes per capita on average FBI hate crimes per capita, and saved the residuals. I then plotted the residuals against Hillary vote share (shown below). Note that the residual for a particular state quantifies how many more SPLC hate crimes that state had relative to the number that would have been expected based on previous years' hate crimes (as reported by the FBI). The plot is shown below:

There is a relatively strong positive correlation. In states with more Hillary votes, there were more hate crimes reported by the SPLC relative to the number that would have been expected based on previous years' hate crimes.

Please note that this analysis in no way proves that the SPLC were unable to exclude a large number of hoaxes or false reports, or even a small number. Rather, it is merely suggestive. Comments are welcome, and all data are available upon request.