Parametric Statistics: Exploring Assumptions.
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1 Parametric Statistics: Exploring Assumptions
2 Reading - Field: Chapter 5
3 R Packages Used in This Chapter For this chapter, you will use the following packages: Start Rcmdr install.packages( car ); install.packages( ggplot2 ); install.packages( pastecs ); install.packages( psych ); library(car); library(ggplot2) library(pastecs) library(psych) NOTE: red font indicates RCmdr dependencies
4 Exploring Assumptions Assumptions of parametric tests based on the normal distribution Aim of this chapter: Quantify the assumption of normality o Graphical displays o Skew o Kurtosis o Normality tests Quantify the homogeneity of variances (when dealing with 2 or more samples): Levene s test
5 Assessing Normality We do not have access to sample the entire biological population, so we test observed data 1) Central Limit Theorem If N < 25, sampling distribution rarely normal 2) Graphical Displays Histogram Q-Q plot 3) Skewness / Kurtosis (point estimate +/- SE) Do they overlap with 0? (normal distribution)
6 Assessing Normality 4) Performing Statistical Tests Shapiro Wilk Test Tests if data differ from a normal distribution Significant = non-normal data Non-Significant = Normal data Levene s Test (comparing 2 or more samples) - Tests if the data distributions have equal variances Significant = different variances Non-Significant = equal variances
7 Assessing Normality - Graphically Characteristics of Normal Distributions Unimodal, Symmetrical, Bell-shaped
8 Assessing Normality - Graphically Comparing observations against a cumulative normal distribution (same mean and S.D.)
9 sample sample Assessing Normality - Graphically theoretical theoretical A percentile is the proportion of cases (observations) that fall below a certain value. Each observed percentile compared to the percentile that the value would have in a normal distribution.
10 Example: Festival Data Set Biologist worried about potential health effects of music festivals. Measured hygiene of 810 concert-goers over the three days of a music festival. Hygiene measured using standardized index (from 0 to 4): 0 = you smell terribly 4 = you smell beautifully Import Download Festival Data (DownloadFestival.xlsx) For ease of use, rename the Data Set Festival > Festival <- DownloadFestival
11 histogram density Explore Data Graphically: RCmdr day1 day2 day3
12 Graphs in Rcmdr Quantiles Normal Distribution is the Default Identifies Max / Min as Default Graphically compares an observed (empirical) distribution (points) with a chosen theoretical expectation (line) Identify Points: Automatic or Interactively
13 Graphs in Rcmdr Quantiles day1 The solid red line is the expected pattern a normal distribution with the same mean and SD and the sampled data. Points outside of the dashed line envelope suggest significant deviations
14 Graphs in Rcmdr Quantiles day 2 day 3 Note: The straight line represents the expected pattern for a normal distribution
15 Explore Festival Data Set We can also explore the summary statistics describing the three datasets (day1, day2, day3) using RCmdr: > numsummary(festival[,c("day1", "day2", "day3"), drop=false], statistics=c("mean", "sd", "IQR", "quantiles", "skewness", "kurtosis"), quantiles=c(0,.25,.5,.75,1), type="2")
16 Explore Festival Data Set We can also explore the summary statistics describing the three datasets (day1, day2, day3) using RCmdr: NOTE: multiple datasets can be analyzed at once What statistics would you use to assess data normality?
17 Explore Festival Data Set Exploring the summary statistics describing the three datasets (day1, day2, day3) using RCmdr: > numsummary(festival[,c("day1", "day2", "day3"), drop=false], statistics=c("mean", "quantiles", "skewness", "kurtosis"), quantiles=c(.5), type="2") mean skewness kurtosis 50% n NA day day day
18 Further Explore Festival Data Set Exploring additional datasets using other functions: describe() function in psych package > describe(festival$day1) vars n mean sd median skew kurtosis trimmed mad min max range se
19 Further Explore Festival Data Set Exploring additional datasets using other functions: stat.desc() function in psych package > stat.desc(festival$day1, basic = FALSE, norm = TRUE) basic argument: Basic statistics included if TRUE (Note: FALSE is the default) norm argument: Statistics relating to normal distribution included if TRUE (Note: FALSE is the default)
20 Further Explore Festival Data Set > stat.desc(festival$day1, basic = FALSE, norm = TRUE) median e+00 SE.mean e-02 var e-01 mean e+00 C.I.mean e-02 std.dev e-01 coef.var e-01
21 Further Explore Festival Data Set > stat.desc(festival$day1, basic = FALSE, norm = TRUE) skewness skew.2se e e+01 kurtosis kurt.2se e e+02 skew.2se: Skew divided by 2 SE kurtosis.2se: Kurtosis divided by 2 SE How can we interpret these results? Z= (observed value theoretical value) / (SE of value)
22 Further Explore Festival Data Set skewness skew.2se e e+01 kurtosis kurt.2se e e+02 skew.2se: Skew divided by 2 SE kurtosis.2se: Kurtosis divided by 2 SE What values are needed to have a significant skew / kurtosis significant? (Different from 0)
23 Further Explore Festival Data Set skew.2se = 5.14 (observed skew) / 2 SE kurtosis.2se = 492 (observed skew) / 2 SE Are skew / kurtosis significant? (Different from 0) YES Rules of thumb to assess significance: skew.2se kurtosis.2se P value ABS > 0.98 < 0.05 ABS > 1 < 0.04 ABS > 1.29 < 0.01 ABS > 1.65 < 0.001
24 Testing Data Normality > stat.desc(festival$day1, basic = FALSE, norm = TRUE) NOTE: Because norm argument set to TRUE, stat.desc provided normality test normtest.w e-01 normtest.p e-37 Test Statistic P value Is this distribution different from a normal distribution? How do I know that? YES P < 0.05 NOTE: Null Hypothesis is that data are normal
25 Testing Data Normality > shapiro.test(festival$day1) Shapiro-Wilk normality test data: Festival$day1 W = , p-value < 2.2e-16 Is this distribution different from a normal distribution? How do I know that? YES P < 0.05 NOTE: Null Hypothesis is that data are normal
26 Testing Data Normality Shapiro-Wilk normality test data: Festival$day2 W = , p-value = 1.282e-11 Shapiro-Wilk normality test data: Festival$day3 W = , p-value = Is day2 different from a normal distribution? How do I know that? YES (P < 0.05) Is day3 different from a normal distribution? How do I know that? YES (P < 0.05)
27 histogram density Graphical Data Exploration: RCmdr day2 day3 Diagnostics: Lack of Symmetry Long tails Mean > Median Positive Skew Positive kurtosis
28 Summary Statistics & Quantiles mean skewness kurtosis 50% n day day day 2 day 3
29 Rule of Thumb (Z scores) skewness2.se kurtosis.2se Day Day Significant Results day 2 day 3
30 Summary: Normality Indicators of a normal (Gaussian) distribution A. Mean = Median = Mode B. Skewness: measures asymmetry of the distribution. A value of zero indicates symmetry. Symmetry is needed to be a normal distribution. The larger the absolute value the more skewed the distribution. C. Kurtosis: measures the distribution of mass in the distribution. A value of zero indicates a normal distribution. The larger the absolute value the more distorted the distribution.
31 1. Assess Normality Graphically Note: The straight line represents the expected pattern for a normal distribution
32 2. Assess Skew / Kurtosis Calculate probability of observed skew / kurtosis, compared to expectation for normal distribution Use rule of thumb : skew.2se kurtosis.2se P value ABS > 0.98 < 0.05 ABS > 1 < 0.04 ABS > 1.29 < 0.01 ABS > 1.65 < 0.001
33 3. Use Shapiro-Wilk (S-W) Test Specific test developed to test null hypothesis that a given sample (x 1,..., x n ) came from a normally distributed population. Significant = non-normal data Non-Significant = Normal data Shapiro, SS, Wilk, MB An analysis of variance test for normality (complete samples). Biometrika 52:
34 Summary Parametric tests based on normal distributions 3 ways of Checking the assumption of normality Graphical displays: Q-Q plots Skew & Kurtosis: Z scores Normality test: S-W Next Lecture: When and how to correct problems in the distribution of the data Data Transformations Pitfalls and alternatives
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