l Types and Sources of error Sampling error Random nondirectional When a sample is used instead of a census Nonsampling error Directional bias overstating or understating the true population parameter Potential sources > Poor sample design > Poor measurement > Poor instrumentation o 2002 The Wadsworth Group
Types and Sources of Error • Sampling Error – Random, nondirectional – When a sample is used instead of a census • Nonsampling Error – Directional bias overstating or understating the true population parameter – Potential sources: »Poor sample design »Poor measurement »Poor instrumentation © 2002 The Wadsworth Group
Im Types of samples Probability, or Scientific, Samples: Each element to be sampled has a known( or calculable) chance of being selected Simple random Every person has an equal chance of being selected Best when roster of the population exists Systematic Randomly enter a stream of elements and sample ever kth element, Best when elements are randomly ordered, no cyclic variation o 2002 The Wadsworth Group
Types of Samples • Simple random • Systematic • Every person has an equal chance of being selected. Best when roster of the population exists. • Randomly enter a stream of elements and sample every kth element. Best when elements are randomly ordered, no cyclic variation. Probability, or Scientific, Samples: Each element to be sampled has a known (or calculable) chance of being selected. © 2002 The Wadsworth Group