The following table shows the out-performance of the FTSE 100 over the FTSE 250 for every month since 1986 (the year of introduction of the latter index). For example, in January 1986 the FTSE 100 month return was 1.6%, that for the FTSE 250 was 2.6%, and so the out-performance of the FTSE 100 over the FTSE 250 was -1.0 percentage points. Negative values (i.e. those months when the FTSE 250 out-performed the FTSE 100) are highlighted with a blue cell background.
This can also be seen in the chart below which shows the average FTSE 100 out-performance of the FTSE 250 for each of the 12 months since 1986. For example, on average in January the FTSE 100 has under-performed the FTSE 250 Index by 1.5 percentage points.