Here are the results for all 30 years:
On the left are results from my prior blog post, showing September-December returns in years when August has been negative. The average return in those 13 Augusts was -4.7% and the average September-December period return was +9.2%, with notably all 13 Sep-Dec periods being positive. I also give the median percentage figures, -3.9% and +6.2% respectively.
On the right are results for the 17 years when August had a positive return. The average return in those 17 Augusts was +2.7% and the average September-December period return was -0.7%. The median percentage figures were +2.0% and +0.6%, respectively.
As I always warn, the usual caveats apply (sample size is limited, past results do not guarantee future results, etc.), but clearly years with down or negative-return Augusts tend to have better subsequent September-December periods than do those years with up or positive-return Augusts.