By memisc 0.99.26.3

Conditional Evaluation of an Expression

Description

The function By evaluates an expression within subsets of a data frame, where the subsets are defined by a formula.

Usage

By(formula,expr,data=parent.frame())

Arguments

formula

an expression or (preferably) a formula containing the names of conditioning variables or factors.

expr

an expression that is evaluated for any unique combination of values of the variables contained in formula.

data

a data frame, an object that can be coerced into a data frame (for example, a table), or an environment, from which values for the variables in formula or expr are taken.

Value

A list of class “by”, giving the results for each combination of values of variables in formula.

Examples

berkeley <- Aggregate(Table(Admit,Freq)~.,data=UCBAdmissions)
(Bres <- By(~Dept,glm(cbind(Admitted,Rejected)~Gender,family="binomial"),data=berkeley))

Dept: A

Call:  glm(formula = cbind(Admitted, Rejected) ~ Gender, family = "binomial")

Coefficients:
 (Intercept)  GenderFemale
      0.4921        1.0521

Degrees of Freedom: 1 Total (i.e. Null);  0 Residual
Null Deviance:           19.05
Residual Deviance: 5.551e-15         AIC: 15.71
---------------------------------------------------------------------------
Dept: B

Call:  glm(formula = cbind(Admitted, Rejected) ~ Gender, family = "binomial")

Coefficients:
 (Intercept)  GenderFemale
      0.5337        0.2200

Degrees of Freedom: 1 Total (i.e. Null);  0 Residual
Null Deviance:           0.2586
Residual Deviance: 0         AIC: 14.27
---------------------------------------------------------------------------
Dept: C

Call:  glm(formula = cbind(Admitted, Rejected) ~ Gender, family = "binomial")

Coefficients:
 (Intercept)  GenderFemale
     -0.5355       -0.1249

Degrees of Freedom: 1 Total (i.e. Null);  0 Residual
Null Deviance:           0.751
Residual Deviance: -4.929e-14        AIC: 16.9
---------------------------------------------------------------------------
Dept: D

Call:  glm(formula = cbind(Admitted, Rejected) ~ Gender, family = "binomial")

Coefficients:
 (Intercept)  GenderFemale
    -0.70396       0.08199

Degrees of Freedom: 1 Total (i.e. Null);  0 Residual
Null Deviance:           0.2979
Residual Deviance: 1.883e-13         AIC: 16.65
---------------------------------------------------------------------------
Dept: E

Call:  glm(formula = cbind(Admitted, Rejected) ~ Gender, family = "binomial")

Coefficients:
 (Intercept)  GenderFemale
     -0.9570       -0.2002

Degrees of Freedom: 1 Total (i.e. Null);  0 Residual
Null Deviance:           0.9904
Residual Deviance: 1.821e-14         AIC: 15.6
---------------------------------------------------------------------------
Dept: F

Call:  glm(formula = cbind(Admitted, Rejected) ~ Gender, family = "binomial")

Coefficients:
 (Intercept)  GenderFemale
     -2.7697        0.1889

Degrees of Freedom: 1 Total (i.e. Null);  0 Residual
Null Deviance:           0.3836
Residual Deviance: -4.863e-14        AIC: 13.83

# The results all have 'data' components
str(Bres[[1]]$data)

'data.frame':        2 obs. of  4 variables:
 $ Gender  : Factor w/ 2 levels "Male","Female": 1 2
 $ Dept    : Factor w/ 6 levels "A","B","C","D",..: 1 1
 $ Admitted: num  512 89
 $ Rejected: num  313 19

attach(berkeley)
(Bres <- By(~Dept,glm(cbind(Admitted,Rejected)~Gender,family="binomial")))

Dept: A

Call:  glm(formula = cbind(Admitted, Rejected) ~ Gender, family = "binomial")

Coefficients:
 (Intercept)  GenderFemale
      0.4921        1.0521

Degrees of Freedom: 1 Total (i.e. Null);  0 Residual
Null Deviance:           19.05
Residual Deviance: 5.551e-15         AIC: 15.71
---------------------------------------------------------------------------
Dept: B

Call:  glm(formula = cbind(Admitted, Rejected) ~ Gender, family = "binomial")

Coefficients:
 (Intercept)  GenderFemale
      0.5337        0.2200

Degrees of Freedom: 1 Total (i.e. Null);  0 Residual
Null Deviance:           0.2586
Residual Deviance: 0         AIC: 14.27
---------------------------------------------------------------------------
Dept: C

Call:  glm(formula = cbind(Admitted, Rejected) ~ Gender, family = "binomial")

Coefficients:
 (Intercept)  GenderFemale
     -0.5355       -0.1249

Degrees of Freedom: 1 Total (i.e. Null);  0 Residual
Null Deviance:           0.751
Residual Deviance: -4.929e-14        AIC: 16.9
---------------------------------------------------------------------------
Dept: D

Call:  glm(formula = cbind(Admitted, Rejected) ~ Gender, family = "binomial")

Coefficients:
 (Intercept)  GenderFemale
    -0.70396       0.08199

Degrees of Freedom: 1 Total (i.e. Null);  0 Residual
Null Deviance:           0.2979
Residual Deviance: 1.883e-13         AIC: 16.65
---------------------------------------------------------------------------
Dept: E

Call:  glm(formula = cbind(Admitted, Rejected) ~ Gender, family = "binomial")

Coefficients:
 (Intercept)  GenderFemale
     -0.9570       -0.2002

Degrees of Freedom: 1 Total (i.e. Null);  0 Residual
Null Deviance:           0.9904
Residual Deviance: 1.821e-14         AIC: 15.6
---------------------------------------------------------------------------
Dept: F

Call:  glm(formula = cbind(Admitted, Rejected) ~ Gender, family = "binomial")

Coefficients:
 (Intercept)  GenderFemale
     -2.7697        0.1889

Degrees of Freedom: 1 Total (i.e. Null);  0 Residual
Null Deviance:           0.3836
Residual Deviance: -4.863e-14        AIC: 13.83

detach(berkeley)