Hello - I’m interested in how economic conditions have changed over time from 2000 to 2015, so I’m using the geographic crosswalk files as specified with GISJOINER and the interpolation weights. The instructions are pretty straightforward when my variable of interest is a count (e.g., # of foreign born individuals in the zone of interest), but what about for medians? (e.g., median household income, medina home value). Can I still use the weights in the same way as I would for counts? Thank you!
Using the NHGIS crosswalks for medians is not as simple. (This is the main reason we haven’t yet extended NHGIS standardized time series to include medians.) E.g., if you applied the crosswalk weights to median household income in exactly the same way as with household counts, then if a target unit is estimated to contain 5% of a source unit’s households, you’d end up allocating 5% of the source median income to that target unit, say, $2,500 out of $50,000. But of course, that’s a bit nonsensical! It’d be better to assume those 5% of households have the same median income as the whole source unit, rather than 5% of the median. The crosswalks, however, don’t yet support making that kind of assumption!
The simplest alternatives right now are one of the following:
- Use means instead of medians, and apply crosswalks separately for each mean’s numerator and denominator. E.g., to compute per capita income, you could estimate “aggregate income” and “total population” separately using the crosswalk weights, and then divide one by the other.
- Start with a table of counts broken down by the value of interest, e.g., housing units by home value, and use the crosswalk to estimate each count in the table. Then estimate the median from the frequency distribution, for which there are various online guides.
Thanks so much, makes sense!