For a client, CIRCLE is preparing an index of American civic engagement that will aggregate the trends in more than 40 indicators over the past thirty years. Our various trends begin and end at different times, depending on the whims of survey designers and changes in actual behavior. Therefore, it wouldn’t work to build an index by averaging the indicators for each year. If an activity that happens to be common (such as wearing a political button or sticker) is suddenly included in the available surveys, then the whole index would jump up arbitrarily. Likewise, we couldn’t add measures of new and relatively rare forms of engagement, such as writing blogs, without arbitrarily lowering the index.
Therefore, I’m proposing that we measure the percentage-point change in each indicator compared to the baseline year in which it was introduced. For instance, watching the TV news enters the National Election Survey in 1984 at 72% and rises to 85% in 1986. I don’t include it at all in 1984, but count it as 13 points in 1986. The index is thus a weighted average difference in all the indicators compared to when they began. Adding blogging doesn’t lower the index, but as blogging becomes more popular, the growth tends to raise the index.
I don’t know if this is an original method–probably not. I also don’t know for sure if it’s a good idea. I think it has some advantages, especially for measuring a category (such as “civic engagement”) that is likely to evolve over time.