canberrabirds [Top] [All Lists]

## Pied Currawong: the facts on abundance trends from GBS data

 To: "martin butterfield" <> Pied Currawong: the facts on abundance trends from GBS data "Philip Veerman" <> Sun, 30 Mar 2008 16:58:43 +1000
Martin,

Sorry you are wrong. My comments are correct but have a look at Appendix 8 of the book on page 121, which is as copied below. How is that consistent with your wrong accusation of my book being "without any measure of significance"? The many reviews of the GBS Report (see Appendix 9) do not mention a lack of statistical rigour therein.

The fact remains that applying analyses to the regressions of the species trends graphs is simply beyond the scope of the size of the book and the intended non-technical audience, not all of whom have studied statistics. Besides as the book clearly states bird populations did not start in 1981 nor finish in 2002. There is no practical reason to particularly assume that the trends for any one species would follow a linear trend of increase or decrease throughout that time. However in some aspects that still forms a useful model. The other point to be aware of is that many species, such as the Pied Currawong, the seasonal variations in abundance is far greater than the long term changes. When statistical models asses this, the perceive the seasonal variations as random, rather than regular and therefore do not show the trends as significant.

Extract follows (I hope it does not lose its formatting and comes through as a neat table).

The data shown in Figures: 4, 8, 23, 26, 28, 29 & 30 represent information that can be assessed with regression analysis. This method can calculate the line of best fit, which is included in these graphs (not Figure 23). It can also demonstrate how well they fit to this straight line. In each case the X & Y axes are as shown in the graph. The regression line is given by the standard formula of Y = a + bX. The values of a (intercept on Y where X = 0) and b (slope of the line), n (the sample size), f value from the Analysis of variance (ANOVA) table and the probability p significance of that f value are given, with a comment. High f values and low p values mean the regression line is a good fit to the data. Note that the p column includes powers of ten, so that 1.46, E-04 is 0.000146. Common practice gives p < 0.05 as significant.

 Figure Page a b n f p Significance 4 25 0.687 0.0094 20 22.974 1.46, E-04 High 8 26 -14.343 43.6632 294 8439.616 1.6, E-217 Huge 23 40 25.473 0.3877 1316 167.952 3.10, E-36 Huge 26 41 7.156 0.9778 294 2264.463 1.34, E-139 Huge 28 42 120.030 0.0070 21 11.148 0.0034 High 29 42 13.959 0.2071 21 90.967 1.12, E-08 Very high 30 43 82.665 -0.0025 21 1.406 0.2503 Not
 Current Thread Pied Currawong: the facts on abundance trends from GBS data, Philip Veerman Pied Currawong: the facts on abundance trends from GBS data, martin butterfield Pied Currawong: the facts on abundance trends from GBS data, Philip Veerman <=