Intro to Research Methods in Criminology Exam 2 Review Sampling Theory and Techniques Sampling A procedure by which we select a sub population of elements from a total population o GOAL To infer the characteristics of a population through measurements of a subpopulation o Related Terms Element A single member of a population or the unit of things chosen for a sample e g people events objects Sample Some part of a larger body of elements specifically selected through a variety of possible methods to represent the whole i e A group of elements that are selected for study Population All possible elements that can be chosen from based on some designated set of specifications The researcher specifies the specifications or criteria of a population e g All inmates released from prison in Florida in 2000 all youth who exited high school in the U S in 2005 all youth adjudicated as a delinquent in California in 2008 Sampling is Easier Quicker and Less Costly than measuring the population as a whole It is not Financially or Practically possible to take measurements on an entire population o Why we sample o Two Broad Types of Sampling Probability Sampling Samples that permit estimation of the likelihood of each element of the population being selected in the sample One CAN SPECIFY for each element of the population the probability that it will be included in the sample Simplest Case Each element has the same probability of inclusion in the sample You can determine the size of the sample needed for a given level of accuracy o i e How different your results might be compared to the population Non Probability Sampling estimating the probability or likelihood that each element has of being included in the sample NO WAY of knowing or that every element has SOME chance NO ASSURANCE of being included Only useful for exploratory research or analysis of extreme cases Usually done for Convenience Economy or it is the Only Method Possible o Types of Probability Sampling Simple Random Sample Each Element in the population has the same probability of being selected in the sample Must have a list of all elements in the population e g all convicted felons in Florida all high school students in the U S etc Process o Determine how many cases you want or can sample o Then use a random number table or random number generator to select the cases for measurement o Now you can select cases randomly with statistical software packages such as SPSS or SAS Systematic Sampling A special type of Simple Random Sampling Need to make sure there is no order to the listing of cases in your population Process o Select every nth case as long as there is no pattern to how the cases are ordered o Randomly select which case you begin with to make it a random sample EXAMPLE If you were studying a population of married couples and the cases were listed in order of husband wife husband wife etc and you selected every even numbered case you would only select females Stratified Random The population is divided into two or more strata based on relevant criteria Strata mutually exclusive groups within your population Process o A simple random sample is taken from each stratum and the sub samples are then joined to form the total sample EXAMPLE You might sample by crime type or on some offender characteristic such as gender age age or race You want to select within strata that you believe are relevant to your study Cluster Sampling Cluster sampling is less time consuming and requires fewer resources than simple random sampling because the geographical region is smaller and lists of the population shorter however you have more error with cluster sampling You may have an atypical cluster that causes more error than an atypical element from a simple random sample Process o Population is divided into clusters and then a probability sample of cluster is selected for study o The researcher then samples the elements within each of the clusters chosen o Types of Non Probability Sampling Accidental Samples No set basis for case selection Take cases that fall to into your hand Like using this class in our seriousness of crime and punishment study No way of knowing the level of bias introduced in Accidental Samples Quota Sampling A non probability stratified sample Judgmentally select elements to fit a distribution equal to what the population s distribution is on key variables It is difficult to sample on more that a few dimensions Also the actual proportions in the population may differ from what you believed it to be Purposive Samples Also called judgmental sampling The researcher selects cases he or she considers representative of the population they want to generalize about based on informed opinion One strategy is to pick cases that are judged to be typical of the population you are studying Snowball Sampling Choosing a first subject then another one based on an introduction from the first etc If you are studying homeless people gang members persons who visit brothels you could start with one person in these types of populations asking them for a name of another person etc Two forms of Stratified Random Sampling 1 Proportionate Stratified Sample subjects are chosen in roughly the same ratio that they exist in the population a If you have a stratum of crime types and robbery is 15 of your population you would randomly select within your robbery stratum and it would comprise 15 of your total sample 2 Disproportionate Stratified Taking a larger than proportionate number of certain groups to assure the appearance of a sufficient number of cases for comparative purposes of a group that is small in the population a For example one may need to oversample on low frequency crime types such as murderers Two Types of Cluster Sampling 1 Single Stage Cluster Sampling Select one set of clusters a For example you are studying sentencing policy in Florida Could select a random sample of the 20 judicial circuits in the state 2 Multi Stage Cluster Sampling When you sample some number of clusters within broader clusters a For example each judicial circuit has one or more counties that comprise that circuit You might sample some number of counties within some number of the 20 judicial circuits in Florida Two Terms Related to Sampling Population Parameter This is the true value or statistic for an entire population o Statistic o For Example If you are measuring the level of Fear of Crime in a In this context can be an average median correlation etc community
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