Contents: Introduction/Data Set Up Describing Quantitative Variables Describing Qualitative Variables




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SAS Procedures for Common Statistical Analyses

Contents:

  1. Introduction/Data Set Up

  2. Describing Quantitative Variables

  3. Describing Qualitative Variables

  4. Two-Sample Tests (Independent Samples)

  5. Completely Randomized Design (1-Way ANOVA)

  6. Randomized Block Design

  7. 2-Factor ANOVA

  8. Chi-Square Tests

  9. Linear Regression

  10. Correlation

  11. Generalized Linear Models

  1. Logistic Regression

  2. Poisson Regression

  3. Negative Binomial Regression



  1. Introduction/Data Set-Up

For all descriptions, we will have datasets where each line represents an individual case, and there are 3 quantitative variables: X, Y, Z measured; and 2 qualtative variables: A, B given, unless otherwise noted.

DATA ONE;

INPUT X Y Z A B;

CARDS;

Data Here


;

RUN;

NOTE: All procedures can be done separately for all levels of one or more factors, and specifically for only cases that meet some criteria.



Analysis Conducted separately for all levels of Factor A:

Data step


RUN;

PROC SORT; BY A; RUN;

PROC PROCNAME;

BY A;

Other PROC Statements


RUN;

Analysis Conducted only on cases where (say) A=1:

Data step


RUN;

PROC PROCNAME;

WHERE A=1;

Other PROC Statements


RUN;

  1. Describing Quantitative Variables


Dataset contains

  • 3 quantitative variables: X,Y,Z


  • 2 qualitative Factors: A,B



Basic Statistics: PROC MEANS

Default: Mean, Standard Deviation, Minimum, Maximum


For all cases:

PROC MEANS;

VAR X Y Z;

RUN;

For cases separately by Factor A:

PROC MEANS;

CLASS A;

VAR X Y Z;

RUN;

For cases separately by combinations of Factors A & B:

PROC MEANS;

CLASS A B;

VAR X Y Z;

RUN;

Full-blown Summary: PROC UNIVARIATE


Default: Moments, SS, CV, SEM, Median, IQR, Tests for Location (=0: t-test, Median=0: Sign, Signed-Rank tests), Quantiles, Extreme Observations

PROC UNIVARIATE;

VAR X Y Z;

RUN;

  1. Describing Qualitative Variables

Note: Dataset need not contain quantitive variables X, Y, Z; but does contain qualitative responses A and B.

Frequency Tabulation for a Single Qualitive Response (A):

PROC FREQ; TABLES A; RUN;

Frequency Cross-Tabulation for Pair of Qualitive Responses (A,B):

PROC FREQ; TABLES A*B; RUN;

NOTE: In many instances you may wish to re-produce and further analyze data previously published in a contingency table. Then each “case” is a cell in the table, and you will include a count for each cell.

DATA ONE;

INPUT A B NUMCASE;

CARDS;

1 1 25


1 2 32

2 1 17


2 2 42

;

RUN;



PROC FREQ; TABLES A*B; WEIGHT NUMCASE; RUN;

  1. 2-Sample tests (Independent Samples)

For this case, assume Factor A has 2 levels, and X is our response variable.

TTEST Procedure: H0: 1-2 = 0 versus HA: 1-2  0


The procedure will conduct the t-test based on the assumptions of equal and unequal variances, as well as the F-test for equal variances to guide you to which analysis to use.

PROC TTEST;

CLASS A;

VAR X;

RUN;

NPAR1WAY Procedure: H0: M1-M2 = 0 versus HA: M1-M2 0

PROC NPAR1WAY WILCOXON;

CLASS A;

VAR X;

RUN;


  1. Completely Randomized Design (1-Way ANOVA)

Statistical Model: Y = i + ij = i + ij i=1,…,a j=1,…,ni

Let Factor A represent the treatment factor and Y be the response variable. The dataset AOVOUT will contain the original dataset and residuals (with variable name E).

ANOVA F-test, Levene’s Test for Equal Variance and Bonferroni/Tukey Comparisons



PROC GLM;

CLASS A;

MODEL Y = A;

MEANS A / BON TUKEY HOVTEST;

OUTPUT OUT=AOVOUT R=E;

RUN;

Kruskal-Wallis H-Test (Nonparametric)
PROC NPAR1WAY WILCOXON;

CLASS A;

VAR Y;

RUN;


  1. Randomized Block Design

Statistical Model: Y = i + bj + ij = i +bj + ij i=1,…,a j=1,…,b

Let A represent the treatment factor, B represent the blocking factor, and Y be the response variable. The dataset AOVOUT will contain the original dataset and residuals (with variable name E).

ANOVA F-test and Bonferroni/Tukey Comparisons



PROC GLM;

CLASS A B;

MODEL Y = A B;

MEANS A / BON TUKEY;

OUTPUT OUT=AOVOUT R=E;

RUN;

Friedman’s Test (Nonparametric)

PROC FREQ;

TABLES B*A*Y / CMH2 SCORES=RANK NOPRINT;

RUN;

Statistic and P-Value are printed by “Row Mean Scores Differ”




  1. 2-Factor ANOVA

Statistical Model:

Y = i + j +()ij + ijk i=1,…,a j=1,…,b k=1,…,n

The dataset AOVOUT will contain the original dataset and residuals (with variable name E).

Additive Model – No Interaction


PROC GLM;

CLASS A B;

MODEL Y = A B;

MEANS A B / BON TUKEY;

OUTPUT OUT=AOVOUT R=E;

RUN;

Model With Interaction


PROC GLM;

CLASS A B;

MODEL Y = A B A*B;

MEANS A B / BON TUKEY;

OUTPUT OUT=AOVOUT R=E;

RUN;

  1. Chi-Square Test


Cases are classified on two qualitative variables: A and B

Want to test whether the classifications are independent (or that the conditional distribution of variable B is the same for every level of A).

PROC FREQ;

TABLES A*B / CHISQ EXPECTED;

RUN;

When measures of association (and tests of significance) are desired instead of the Chi-Square test, use:



PROC FREQ;

TABLES A*B / MEASURES;

RUN;


  1. Linear Regression

Simple Linear Regression

Statistical Model: Yi = 0 + 1Xi + i i=1,…,n

The dataset REGOUT will contain the original dataset and residuals (with variable name E).

PROC REG;

MODEL Y = X;

OUTPUT OUT=REGOUT R=E;

RUN;

Multiple Linear Regression (Dataset contains variables X1,…,Xk)

Statistical Model: Yi = 0 + 1X1i +…+ kXkii i=1,…,n

PROC REG;

MODEL Y = X1 X2 … Xk;

OUTPUT OUT=REGOUT R=E;

RUN;

  1. Correlation
Data: Variables Y1,…,Yk
Pairwise Bivariate Correlations

PROC CORR; VAR Y1 … Yk; RUN;
Partial Correlation between Y and Z, Controlling for X

PROC CORR; VAR Y Z; PARTIAL X; RUN:

  1. Generalized Linear Models


  1. Logistic Regression


Statistical Model: Y is a binary outcome:



PROC GENMOD;

MODEL Y = X / DIST=BIN LINK=LOGIT;

RUN;
  1. Poisson Regression


Statistical Model: Y is a count outcome:

Yi ~ Poisson(i) log(i) = 0 + 1XI E(Yi) = (i) V(Yi) = i

PROC GENMOD;

MODEL Y = X / DIST=POI LINK=LOG;

RUN;
  1. Negative Binomial Regression


Statistical Model: Y is a count outcome:

Yi ~ NB(i,k) log(i) = 0 + 1Xi E(Yi) = (i) V(Yi) = i + (i2/k)

PROC GENMOD;

MODEL Y = X / DIST=NB LINK=LOG;

RUN;


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