Nominal
Level of measurement that is characterized by data that consist of names, labels, or categories only. The data cannot be arranged in an ordering scheme (such as low to high).
Ordinal
Data are at the ordinal level of measurement if they can be arranged in some order, but differences between values either cannot be determined or are meaningless.
Interval
The interval level of measurement is like the ordinal level, with the additional property that the difference between any two data values is meaningful.
Ratio
The ratio level of measurement is the interval level with the additional property that there is a natural 0 starting point (where 0 indicates that none of the quantity is present).
Cross-Sectional Study
Data are observed, measured, and collected at one point in time.
Retrospective
Data are collected from the past through examination of records, interviews, and so on.
Prospective
Data are collected in the future from groups sharing common factors.
Use Permutations formula when objects must be in a specific order
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Unbiased Estimators
Mean, variance, proportion
Biased Estimators
Median, range, standard deviation