Recovery Plan for the Northern Spotted Owl - DRAFT

Table A.3. Powera for various designs of the roadside surveys.Number ofStationsProportion of Stations Surveyed Every Fourth YearbSurveyedper Year 0.00 0.20 0.40 0.60 0.80 1.00200 0.32 0.33 0.34 0.35 0.37 0.38400 0.51 0.53 0.54 0.56 0.58 0.60600 0.66 0.68 0.69 0.71 0.73 0.75800 0.77 0.78 0.80 0.81 0.83 0.851,000 0.84 0.86 0.87 0.88 0.90 0.91probability of detecting the trend. In these simulations, the average proportional cThe change in the population was -3.4 percent per year.9 The remaining stations were surveyed every year., .... ... .. .. ................ ............ ~~~~~~~~~~~~~~~~~~......... ...........256stations were required for 80 percent power when all stations were visited eachyear, but only 370 stations were required for the same power when all stationswere visited at 4-year intervals. The lattice design thus requires that moreroutes be selected, but permits a smaller number to be surveyed in each year(to achieve a given power) than a design in which each route is surveyed eachyear.The decision on whether to adopt a lattice design can be postponed until thesecond year of monitoring. At that time, a decision must be made to revisitevery route surveyed in the first year or to temporarily drop some routes andintroduce a corresponding set of new routes. If the new set were spatiallyinterpenetrating with those that were dropped, then the logistics of the programwould not be compromised by this tactic and the geographic dispersionof the sample would remain essentially the same. (Such a spatially interpenetratingdesign on a 4-year cycle is being implemented by the U.S. EnvironmentalProtection Agency in its newly instituted Environmental Monitoring andAssessment Program, EMAP.)Sample size requirementsSample size includes the number of stations (or routes) monitored per year andthe number of years during which monitoring continues. Many factors affectsample size requirements. Among the most important are the sampling designused to collect the data, the desired level of precision or power, and the degreeto which the true trend conforms to the statistical (e.g., linear) model. Atpresent, many of the factors needed to estimate sample size requirements forthe roadside survey method are unknown. As a result, our analyses arepreliminary and undoubtedly will be altered as more information is obtained.Despite these problems, the analyses provide an indication of approximatesample size requirements which will be useful in deciding on the initial specificationsfor the monitoring program.Two analyses of sample size requirements are presented. The first is based ona series of computer simulations in which statistical methods were used tocreate data sets with known underlying trends. Estimates of these trends thenwere calculated and compared to the known, true values. The process wasrepeated with different parameter sets to investigate sample size requirementsand identify factors that affect them. This analysis permitted a detailed investigationof sample size requirements but required an assumption that thecomputer model was realistic. For the second analysis, we obtained data from