2. Power analysis on real ibex trends in GPNP
If the complete trend observed in the ibex count data in GPNP was significant (86.9% of the time intervals), sample counts were able to correctly detect its direction with a sufficient statistical power (fig. 5a) using a number of sectors between 5 (for a 20-year trend) and 28 (for a 5-year trend). To correctly measure the magnitude of the trend, a higher number of sectors was required (fig.5b). The complete trend was detected with an error lower than 2% and in at least 80% of the cases if 10 (for a 20-years trend) to 33 (for a 5-years trend) sectors were surveyed. To detect a trend over 10 years, 15 sectors were needed to correctly assess the direction of the trend and 22 sectors to also detect its magnitude, while for a 20-year trend 10 sectors were sufficient to accurately quantify the trend. Sampling more sectors reduced the number of false, missed and opposing trends (fig.6). Indeed, counting in the full area compared to half of sectors led to false trends declining from 14% to 6%, missed trends from 10% to 3% and opposing trends from 5% to 0%.