I've got spatio-temporal disease data at the county/annual level for 2000-2014. I'm analyzing it to try to pull out temporal variations in disease incidence and was told that I should generate a spatio-temporal variogram. The variogram appears to show a relationship but I'm having a hard time finding out how to interpret the graph (which is attached).
Is this graph showing that disease rates 10-15 years apart are more strongly correlated at <100 km, 200 km, and 500km? and that the temporal correlation drops off sharply at around 8 years apart?
If I remember correctly, the lower the gamma, the greater the autocorrelation? I've looked for a guide on interpretation for a while now but all I can find are guides on how to generate models from this data.
Best Answer
A variogram is a measure of dissimilarity in space/time. The lower is the variogram value, the higher is the correlation between the two pairs of data points.
Thinking about spatio-temporal correlation simultaneously is often tricky. For exploratory analysis purpose, statisticians sometimes tend to separately think about the spatial variogram at zero-time lag and the temporal variogram at zero-spatial lag, which are the two line plots you see when you look at the figure from the front and side planes. Variograms often give you a rough estimate about how your space-time correlation looks like. However, accurate estimates are usually evaluated by fitting a covariance model to your data.
From the figure above, I can draw the following observations: