Parametric and Nonparametric Tests Flashcards
Population Parameter
A number that describes something about an entire group or population
The number in the population is usually high, so it will probably have a normal distribution!
How is the sample size distributed when a parametric test is used?
Normally distributed
Which parameters should be used for skewed data?
Median, IQR (25th percentile, 75th percentile)
Data: Sex number in %
Parametric or Non-parametric
Non-parametric
-bc it is a yes or no answer and no means are used and there represented in categories
-means are continuous and more likely to be represented in a normally distributed curve
What is considered descriptive statistics?
Mean, median, SD, IQR
-used to describe data
What are Inferential Statistics?
Inference = Conclusion (reached based on evidence)
-used to test hypotheses and draw interferences
-Inferential Statistics can be parametric or non-parametric
What are the criteria for Parametric Tests?
-Samples are drawn from a normal distribution
-Variances (SD) in both groups compared are approximately equal (both normally distributed)
-Data are continuous (BP, LDL) !!!
What are non-parametric Tests?
-Statistics that do not test hypotheses concerning population parameters
-assumptions for parametric tests are not met
-they do not assume normal distribution and are distribution-free tests
What type of data is used in non-parametric Tests?
-Nominal data (Male/female, race, dead/alive)
-Ordinal data
-small sample size -> likely to have skewed data
What are common Parametric Tests?
-Student’s T-Test -> often used for continuous data
-Analysis of Variance (ANOVA)
What are common Non-Parametric Tests?
-Mann Whitney U
Nonparametric form of Student’s t-test
-Chi-Square Test / Fisher’s Exact Test
Used for nominal data
Students T-test
-parametric
-comparing two population means (or drug and placebo group)
-most common in the medical literature (more than 50%)
What are the assumptions of the Student t-test?
-Normal distribution
-SD in the 2 groups is approximately equal
-continuous data
-Randomly selected, independent samples
-> There are two different groups (drug vs placebo, compared to dependent in cross-over studies - with the same patients used for drug and placebo -> paired data)
Purpose of Student T-test
-determine the probability of the findings being due to chance
-creating a p-value
What is a t-score?
not on EXAM
-similar to Z-score
-Z-score is the number of SD away from the mean
-t-score is expressed in an estimated SD away from the mean
-Z-scores are stable
-t-scores will vary depending on sample size
Example:
randomized, placebo-controlled, double-blinded study (n=300)
the drug arm of the study reduced LDL by 18±4% and the placebo arm reduced LDL by 13±3%
Student’s t-test was used to compare lipid
reduction, p =0.02
Is the student’s t-test appropriate?
Criteria:
Normally distributed - Y bc the SD is not high relative to the mean
Variances should be approximately equal - Y
Continuous data Y- LDL values are continuous data
Independent groups - Y bc placebo-controlled
Why are Student t-tests not appropriate when comparing multiple groups?
Because the probability of committing a type I error (alpha) sums up each time we compare different groups
Which test is appropriate to test multiple groups?
ANOVA
When to use Non-parametric t-tests?
-When the assumptions for a Student’s t-test (parametric test) are not me
-when we have ordinal data
What are the Non-parametric t-tests?
-Mann Whitney U
-Wilcoxon Rank Sum
-with median
-they tell us whether the medians are different
Which test should be used when the conditions for a parametric t-test are not met?
Mann Whitney U
Which test to use for a nonparametric paired t-test
Wilcoxon Signed Rank test
ANOVA
-ANOVA = Analysis of Variance
-ANOVA is a “group comparison”
-ANOVA is a parametric test
When is an ANOVA test appropriate?
-When comparing the means of three or more groups
-parametric tests
-normally distributed data
