# Shared Flashcard Set

## Details

accp-biostats
biostatistics
31
Pharmacology
Professional
05/15/2011

Term
 Random variables (2)
Definition
 discrete (dichotomous, categorical) continuous
Term
 Discrete Variables (2)
Definition
 Nominal Ordinal can only take a limited number of values within a given range
Term
 Nominal
Definition
 Discrete variable   Classified into groups in an unordered manner &  with no indication of relative severity (sex, mortality, disease state)
Term
 Ordinal
Definition
 discrete variable   ranked in specific order no consistent level of magnitude of difference between ranks (NYHA functional class describes the functional status of pts w/heart failure & subjects classified in increasing order of disability I, II, III, IV) means & SD should not be used with ordinal data in most cases (measure of central tendency)
Term
 Interval
Definition
 Ranked in a specific order with a consistent change in magnitude between units   Zero point is arbitrary (e.g. Fahrenheit)   Continuous variable
Term
 Conditional Variables
Definition
 Interval Ratio   Can take on any value within a given range Sometimes referred to as Counting Variable
Term
 Ratio
Definition
 Data ranked in a specific order with a consistnet change in magnitude between units   Has an absolute zero (e.g. degrees Kelvin, pulse, BP, time, distance)   Continuous variable
Term
 Measures of Central Tendency (3)
Definition
 Mean Median Mode Presenting dat using only measures of central tendency can be misleading without some idea of data spread
Term
 Mean
Definition
 Average Measure of central tendency Sum of all values divided by the total number of values Generally used only for continuous & normally distributed data Very sensitive to outliers  & tend toward the tail which has the outliers  Geometric mean
Term
 Median
Definition
 Measure of central tendency Midpoint of the values when placed in order from highest to lowest half of the observations are above & half below Also called the 50th percentile Can be used for ordinal or continuous data especially good for skewed populations Insensitive to outliers
Term
 Mode
Definition
 Most common value in a distribution Measure of central tendency can be used for nominal, ordinal or continuous data Can be more than 1 mode: bimodal & trimodal Does not help describe meaningful distributions with a large range of calues, each of which occur infrequently
Term
 Measures of Data Spread or variability (3)
Definition
 Standard deviation Range Percentiles
Term
 Standard Deviation
Definition
 Meaure or the variability about the mean (most common measure used to describe this) Square root of the variance (average squared difference of each observation from the mean) returns variance back into original units (non-squared) apply only to continuous data normally or near-normally distributed 68% within 1 SD 95% within 2 SD 99% within 3 SD
Term
 Coefficient of Variation
Definition
 relates the mean & SD   SD/mean x 100%
Term
 Range
Definition
 Difference between the smallest and largest values does not give a tremendous amount of info by itself Easy to compute (simpel subtraction) Size of range is very sensitive to outliers Often reported as the actual values rather than the difference between the 2 extreme values
Term
 Percentiles
Definition
 The point (value) in a distribution in which a value is larger than some % of the other values in the sample 75th percentile (75% of the values are smaller) Does not assume the population has a normal distribution IQR: middle 50% - encompasses the 25th to the 75th percentile
Term
 Population Distributions
Definition
 Discrete Binomial Poisson Normal (Gaussian)
Term
 Normal  (Gaussian) Distribution
Definition
 Most common model for population distribution Symetric or "bell-shaped" frequency distribution median and mean will be approx. equal Frequency ditribution & histograms (visual check look symmetrical & bell-shaped) formal test: Kolmogorov-Smirnov Landmarks for continuous, normally distributed data m: population mean is equal to zero s: population SD is equal to 1 x and s represent the smple mean and SD
Term
 SEM
Definition
 quantifies uncertainty in the estimate of the mean NOT variability in the sample estimates the SD of the means
Term
 Confidence Interval
Definition
 95% CI most common gives and idea of the magnitude of the difference between groups as well as statistical significance if CI contains 0 = no difference between 2 variables (interpreted as not statistically significant: a p-value >/= 0.05) no need to show both 95% CI & p-value odds ratio & relative risk: value of 1 indicates no difference in risk, no statisical difference
Term
 Ho (Null Hypothesis)   (rejected or not rejected)
Definition
 no difference between groups being compared (treatment A = treatment B)  if rejected: statistical significance between groups (unlikely attributable to chance) if accepted (not rejected): no statistically significant difference between groups (any apparent differences may be attributable to chance) not concluding that they are equal
Term
 Ha (alternative hypothesis)
Definition
 opposite of null hypothesis States that there is a difference between groups
Term
 Parametric Tests   Assumptions Examples (3)
Definition
 Assume: data normal or near normal distribution - evaluate whether this is true by comparing mean to median data is continuous measured on an interval or ratio scale Examples: student t-test::: one-sample test: compares mean of the study with population mean two-sample test: compares means of 2 independent samples (may not use multiple t-tests with more than2 groups)   Paired test::: compares the mean difference of paired or matched samples
Term
 Parametric tests   analysis of variance
Definition
 One-way ANOVA: compares means of 3 or more groups Two-way ANOVA: additional factor (e.g. age) added Repeated-Meaures ANOVA: a related samples test
Term
 Nonparametric Tests
Definition
 may also be used for ordinal & continuous data that do not  meet the assumptions of the t-tests or ANOVA Tests: Wilcoson rank sum & Mann-Whitney U-test, compares 2 independent samples (related to a t-test) Kruskal-Wallis one-way ANOVA by ranks, compares 3 or more independent groups (related to a one-way ANOVA) - - post hoc testing
Term
 Nonparametric Tests   (tests for related or paired samples)
Definition
 Sign test & Wilcoson signed rank test: compares 2 matched or paired samples Firedman ANOVA by ranks: compares 3 or more matched/paired groups
Term
 Nominal Data
Definition
 Chi-square (x2) test: compares expected & observed proportions between 2 or more groups:    -   test of independence -   test of goodness of fit   Fisher exact test: specialized version of chi-square test for small groups (cells) containing < 5 observations   McNemar: paired samples                              Mantel-Haenszel: controls for the influence of confounders
Term
 type I error
Definition
 Ho is rejected, when it should not have been   - conclude that there is a statistically significant difference when actually one does not exist   termed a - convention is to set it at 0.05 (5%)
Term
 type II error
Definition
 Ho is not rejected, but should have been   - concluding that no differnce exists when one truly does   termed b  - conventionally set 0.20 to 0.10
Term
 p-value
Definition
 0.05 or 5%
Term
 Power
Definition
 1-b   the probability of making a corret decision when Ho is false the ability to detect differences between groups if 1 actually exists Dependent on: -predetermined a: the risk of error you will tolerate when rejecting Ho - sample size   - the size of the difference between the outcomes wanting to detect - variability of the outcomes being measured
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