Many firms conduct surveys in an annual series. To add a nuance, we have encountered a handful of examples of two surveys per year, such as by Haynes & Boone. Most of them do so each year on the same topic but lapse after three or four years.
The plot below shows all series identified to date as a histogram of how many surveys have appeared in the series. So, for example, five series have continued for thee years (the second column from the left). The plot visualizes a total of 187 research surveys by 32 law firms. Of those firms, two have sponsored at least three series (CMS and DLA Piper). Seven other firms claim at least two series.
Stated differently, just under half of all the research surveys found to date (349) are part of a multi-year series. A handful of the series were interrupted for a year but most of them were continuous.
As can be seen from the left-most column, 16 of the series either carried on for only two years, or 2018 may be their second year and we do not know how long they will continue. On the far right looms the Methuselah of all series, the 13 year run by Dykema Gossett on merger and acquisition activity. Overall, law firms have demonstrated admirable consistency and commitment.