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8-sampling-strategy [2018/08/01 16:13] thorsten [8.3 Selection criteria WITHIN the population] |
8-sampling-strategy [2018/08/01 16:13] thorsten [8.1 Sampling at the primary sampling unit (PSU) level] |
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| ===== 8.1 Sampling at the primary sampling unit (PSU) level ===== | ===== 8.1 Sampling at the primary sampling unit (PSU) level ===== | ||
| The information needed to be collected are:\\ A) Number of PSU in the population (which should have been determined in [[/7-Study-design|section 7]])\\ B) Design prevalence\\ C) Desired confidence\\ D) Desired power\\ E) Sensitivity (Sensitivity at the PSU level equals confidence at the SSU level, if two-stage sampling is used)\\ F) Specificity\\ \\ This information will be used in later steps for sample size calculation. In some cases, the sample size is decided a priori, for instance based on a specific budget. If this is the case, then the information in this section would not need to be collected. Then, in step 8.5 (below) you will find a link to tools to explore different sampling options and carry out a power analysis - identify the achievable confidence using the sample size established a priori.\\ \\ | The information needed to be collected are:\\ A) Number of PSU in the population (which should have been determined in [[/7-Study-design|section 7]])\\ B) Design prevalence\\ C) Desired confidence\\ D) Desired power\\ E) Sensitivity (Sensitivity at the PSU level equals confidence at the SSU level, if two-stage sampling is used)\\ F) Specificity\\ \\ This information will be used in later steps for sample size calculation. In some cases, the sample size is decided a priori, for instance based on a specific budget. If this is the case, then the information in this section would not need to be collected. Then, in step 8.5 (below) you will find a link to tools to explore different sampling options and carry out a power analysis - identify the achievable confidence using the sample size established a priori.\\ \\ | ||
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| ===== 8.2 Sampling at the secondary sampling unit (SSU) level ===== | ===== 8.2 Sampling at the secondary sampling unit (SSU) level ===== | ||
| If one-stage sampling process is applied, then PSU is the only unit to consider (skip this step). For two-stage sampling, the same information should be collected for the SSU level:\\ A) Number of SSUs in the population (which should have been determined in [[/7-Study-design|section 7]])\\ B) Design prevalence\\ C) Desired confidence\\ D) Desired power\\ E) Sensitivity\\ F) Specificity\\ \\ | If one-stage sampling process is applied, then PSU is the only unit to consider (skip this step). For two-stage sampling, the same information should be collected for the SSU level:\\ A) Number of SSUs in the population (which should have been determined in [[/7-Study-design|section 7]])\\ B) Design prevalence\\ C) Desired confidence\\ D) Desired power\\ E) Sensitivity\\ F) Specificity\\ \\ | ||
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| ===== 8.3 Selection criteria WITHIN the population ===== | ===== 8.3 Selection criteria WITHIN the population ===== | ||
| In [[/3-Target-population|section 3]] we discussed criteria to choose the target population. When sampling that target population, however, it is possible that not all animals are reachable (for instance not all units are registered, therefore not all are available in the sampling frame). So here a designer should consider whether, WITHIN the target population, there are any criteria for the selection of animal/units to target which are NOT related to sampling (which is discussed further below).\\ \\ Criteria for selection could be for instance logistic/convenience (this component focuses on a particular sector of the population because they are easier to sample or more accessible; higher probability of infection; higher probability of showing clinical signs; feasibility of detection (the diagnostic tests available can only be used in animals above a certain age, or non-vaccinated animals); or higher severity of consequences in case of infection, which is the case for instance when surveillance is focused on breeder animals.\\ \\ | In [[/3-Target-population|section 3]] we discussed criteria to choose the target population. When sampling that target population, however, it is possible that not all animals are reachable (for instance not all units are registered, therefore not all are available in the sampling frame). So here a designer should consider whether, WITHIN the target population, there are any criteria for the selection of animal/units to target which are NOT related to sampling (which is discussed further below).\\ \\ Criteria for selection could be for instance logistic/convenience (this component focuses on a particular sector of the population because they are easier to sample or more accessible; higher probability of infection; higher probability of showing clinical signs; feasibility of detection (the diagnostic tests available can only be used in animals above a certain age, or non-vaccinated animals); or higher severity of consequences in case of infection, which is the case for instance when surveillance is focused on breeder animals.\\ \\ | ||