Regional Credibility Measures

Although the BBS provides a huge amount of information about regional population change for many species, there are a variety of possible problems with estimates of population change from BBS data. Small sample sizes, low relative abundances on survey routes, imprecise trends, and missing data all can compromise BBS results. Often, users do not take these problems into account when viewing BBS results, and use the results inappropriately.

To provide some guidance to interpretation of BBS data, we have implemented a series of checks for some attributes that we view as cause for caution in interpretation of BBS results. We categorize BBS data in 3 credibility categories:

This category reflects data with an important deficiency. In particular:

  1. The regional abundance is less than 0.1 birds/route (very low abundance),
  2. The sample is based on less than 5 routes for the long term, or is based on less than 3 routes for either subinterval (very small samples), or
  3. The results are so imprecise that a 5%/year change would not be detected over the long-term (very imprecise).
This category reflects data with a deficiency. In particular:
  1. The regional abundance is less than 1.0 birds/route (low abundance),
  2. The sample is based on less than 14 routes for the long term (small sample size)
    3. ,
  3. The results are so imprecise that a 3%/year change would not be detected over the long-term (quite imprecise), or
  4. The sub-interval trends are significantly different from each other (P less than 0.05, based on a z-test). This suggests inconsistency in trend over time).
This category reflects data with at least 14 samples in the long term, of moderate precision, and of moderate abundance on routes.

Notes:

  1. Even data falling in the category may not provide valid results. There are many factors that can influence the validity and use of the information, and any analysis of BBS data should carefully consider the possible problems with the data.
  2. We are occasionally asked to identify which deficiency is causing the flag. However, the point of the codes is to provide a quick and simple set of cautions to users, and we are resisting the notion of setting up a complicated series of codes. To determine why the code exists, look at the results. All of these deficiencies (abundances, precisions, etc) will be evident from the results we present.