tailieunhanh - Statistical Methods in Medical Research - part 9

Lấy mẫu nhiều tầng Trong phương pháp này, khung lấy mẫu được chia thành một dân số của `đơn vị mẫu đầu tiên giai đoạn, trong đó một 'giai đoạn' mẫu được lấy. Điều này thường sẽ là một mẫu ngẫu nhiên đơn giản | 654 Statistical methods in epidemiology N where Pi is the estimated prevalence in the zth stratum. The variance of -ft- is given by the equivalent of in which we shall drop the terms fi as being small VarH E n tMl - M The values of var for the three allocations are as follows Allocation A B c var 0-000714 0-000779 0-000739 As would be expected the lowest variance is for A and the highest for B the latter being only a little lower than the variance for a random sample. Multistage sampling In this method the sampling frame is divided into a population of first-stage sampling units of which a first-stage sample is taken. This will usually be a simple random sample but may be a systematic or stratified sample it may also as we shall see be a random sample in which some first-stage units are allowed to have a higher probability of selection than others. Each sampled first-stage unit is subdivided into second-stage sampling units which are sampled. The process can continue through as many stages as are appropriate. There are two main advantages of multistage sampling. First it enables the resources to be concentrated in a limited number of portions of the whole sampling frame with a consequent reduction in cost. Secondly it is convenient for situations in which a complete sampling frame is not available before the investigation starts. A list of first-stage units is required but the second-stage units need be listed only within the first-stage units selected in the sample. Consider as an example a health survey of men working in a certain industry. There would probably not exist a complete index of all men in the industry but it might be easy to obtain a list of factories which could be first-stage units. From each factory selected in the first-stage sample a list of men could be obtained and a second-stage sample selected from this list. Apart from the advantage of having to make lists of men only within the factories selected at the first stage this procedure .