## 统计代写|生物统计代写Biostatistics代考|Cluster Sampling

In many biomedical studies, a good sampling frame containing the units in the target population is extremely difficult to obtain. In some cases, the units of the population belong to clearly defined groups of units referred to as clusters of units. When a sampling frame that contains a list of clusters is available a random clusterr sample can often be uséd tō obtain a sample that is representative of the target population. For example, in surveying the general health of the people living in a particular town, a sampling frame consisting of the people living in the town would be difficult, if not impossible, to obtain. On the other hand, most towns will have a listing of the housing units within the town. In this case, the people living in the town are grouped in clusters according to housing units.

To use a random cluster sample, the clusters in a target population should be nonoverlapping and exhaustive so that a population unit belongs to only one cluster. Also, when the population units belong to well-defined clusters and a random cluster sample is selected, the cluster random sample will consist of primary and secondary units. The primary units in a cluster sampling plan are the clusters of population units and the secondary units are the units within the clusters. Thus, a primary unit is a sampling unit and a secondary unit is sampling element.

## 统计代写|生物统计代写Biostatistics代考|Systematic Sampling

The final sampling plan that will be discussed is the systematic random sampling plan. A 1 in $k$ systematic sample is a probability sample obtained by randomly selecting one sampling unit from the first $k$ units in the sampling frame and every $k$ th unit thereafter. A systematic random sample is one of the most cost-effective and convenient sampling plans when the population units are randomly dispersed over the sampling frame. Systematic random sampling is also useful in quality control settings. For example, a pharmaceutical company may check the uniformity and quality of the drug tablets it manufactures. The ability to control the manufacturing process by sampling the output is an important aspect of quality control and may be monitored by selecting a systematic random sample of every 100 th tablet produced.
The process for drawing a systematic random sample is outlined below.

The advantages of drawing a systematic random sample are that it is simple to collect, often less expensive than other sampling plans, spreads uniformly over the sampling frame, the statistical analysis of a systematic random sample is the same as the analysis used with a simple random sample, and when the size of the population is not fixed or is being monitored over time, a systematic random sample can be easily implemented without a sampling frame.

The only real disadvantage of using a systematic random sample occurs when a systematic sample isolates unwanted patterns in the population. That is, a systematic random sample may yield a biased sample when there is a periodic or cyclic pattern in the units listed in the sampling frame. Thus, it is critical to avoid the use of a systematic random sample when there could be cyclical patterns in the sampling frame or the sampling process. Cyclical patterns are often due to sampling the units over time.

# 生物统计代考

## 统计代写|生物统计代写Biostatistics代考|Systematic Sampling

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