Reader Bruce Hall comments in his defense of calculating an 18 month change using not seasonally adjusted CPI data, and then 6 months of seasonally adjusted data:
…pgl insists on using 13.3% which is what I wrote on my initial comment regarding this “price level change” based on data not seasonally adjusted. After his “correction of my thinking” I used the Fed’s seasonally adjusted data which was 12.6% from January 2021 to June 2022. Of course, we all know that the end points of seasonally adjusted data are subject to revision as the seasonal factors are recalculated over time. So maybe after several months that number may be 12.4% or 12.8%. But the issue is not the exact percentage change but the general magnitude of that change and how that relates to subsequent (June-December) change. June appears to be the point of discontinuity which is not generally featured in the yr/yr numbers, but has to be gleaned.
Well, I’ve pointed out the hazards of doing 18 month changes on nsa data here; but what about adding nsa and sa data?
Well, here’s an example (from the Scott Walker campaign, in a post of 2016) of adding not only different series, but differently seasonally adjusted (or unadjusted) data.
In particular Wisconsin Governor Walker’s campaign got into trouble when they touted job creation numbers obtained by adding together seasonally unadjusted jobs figures (from what is called the Quarterly Census of Employment and Wages) with seasonally adjusted jobs figures (from the establishment survey) to get a cumulative change in employment. (They did this because QCEW figures lag by many months, while the establishment survey data are more timely). This is shown in Figure 3.
Figure 3: Wisconsin nonfarm private employment from Quarterly Census of Employment and Wages, not seasonally adjusted (blue), private nonfarm private employment from establishment survey, seasonally adjusted (red). Black arrows denote changes over QCEW and establishment survey figures; teal arrows over establishment survey. Source: BLS.
Notice that one can calculate the changes from December 2010 (just before Walker takes office) to March 2012 (the latest QCEW figures available as of December 12, 2012), and then add to the change from March 2012 to October 2012 (the latest establishment figure available as of December 12, 2012). That is, add 89.1 to 6.4 to get 95.5 thousand, close to the 100 thousand figure cited by Governor Walker’s campaign. You can see why Governor Walker’s campaign officials did so – the correct calculation using the change in the establishment survey from December 2010 to October 2012 was only 61.1 thousand.
So, Bruce Hall is, if not in “good” company, in the company of the likes of the Scott Walker campaign.