Does the longevity of a survey series affect the average number of participants in the series? This is likely to be too crude a question, because the target populations of series differ significantly. Then too, firms might modify their questions as the series goes along rather than repeating the same questions, which could affect participation. A series might bring on different co-coordinators or change how it reaches out for participants. If we could control for factors such as these, which might swamp changes in participant numbers arising simply from annual invites, content, and publicity, we could make some headway on the question, but the data for that level of analysis is not available. Also, averaging participant numbers over the years of a survey series may conceal material ups and downs.
Moreover, of greater usefulness to law firms would be knowing whether numbers of participants tend to increase over the life of a series as it becomes better known and more relied on.
We plunge ahead anyway. To start, consider the series that have been sponsored by a law firm for four years or more. We know of 21 as are presented in the plot below. The color coding from the legend at the bottom corresponds to how many surveys have been in the series (some of which are ongoing). The color coding moves from midnight blue for the four-year series to the lightest (yellow) for the longest-running survey (13 years).
As we speculated above, a regression of how many years a survey has been conducted against average participants provides no insight. Other factors than the number of years a survey series has run influence the number of participants more.