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list High School / Advanced Statistics and Data Science I (ABC)

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  • High School / Advanced Statistics and Data Science I (ABC)
  • High School / Statistics and Data Science I (AB)
  • High School / Statistics and Data Science II (XCD)
  • High School / Algebra + Data Science (G)
  • College / Introductory Statistics with R (ABC)
  • College / Advanced Statistics with R (ABCD)
  • College / Accelerated Statistics with R (XCD)
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9.4 Comparing Regression Models to Group Models

Comparing the Height2Group Model and the Height Model

We now know how to specify and fit two different kinds of models: group models (e.g., Height2Group_model) and regression models (Height_model), let’s just think for a bit on what the similarities and differences are between these models.

Symbol Group Model
\(Y_i=b_0+b_1X_i+e_i\)
\(\text{Thumb}_i=b_0+b_1\text{Height2Grouptall}_i+e_i\)
Regression Model
\(Y_i=b_0+b_1X_i+e_i\)
\(\text{Thumb}_i=b_0+b_1\text{Height}_i+e_i\)
\(Y_i\) Thumb length of a student i Thumb length of a student i
\(b_0\) Predicted thumb length when \(\text{Height2Group}_i = 0\))
(mean thumb length for short group)
Predicted thumb length when \(\text{Height}_i=0\)
(y-intercept for regression line)
\(b_1\) Adjustment to predicted thumb length for a tall student
(the mean difference between the two group means)
Adjustment to predicted thumb length for a one-unit increase in height
(the slope of the regression line)
\(X_i\) Height2Group of a student i, coded as 0=not-tall, 1=tall Height of a student i in inches
\(e_i\) Error for student i Error for student i
visualization of the model

A jitter plot of Thumb by Sex with the model predictions in red.

A scatter plot of Thumb by Height with the model predictions in red.


Fitting a Regression Model By Accident When You Don’t Want One

Although R is pretty smart about knowing which model to fit, it won’t always do the right thing. If you code the grouping variable with character strings such as “female” and “male” or “short” and “tall,” R will make the right decision to fit a group model because it knows the variable must be categorical. But if you code the same grouping variable as 1 and 2 (maybe you forget to make it a factor), R may get confused and fit the model as though the explanatory variable is quantitative.

For example, we’ve added a new variable to our Fingers data called SexNum. Here is what the data look like.

 Thumb    Sex SexNum
1    66   male      2
2    64 female      1
3    56 female      1
4    70   male      2
5    52 female      1
6    62   male      2

If you take a look at the variables Sex and SexNum, they have the same information. Students 2, 3, and 5 are in one group and students 1, 4, and 6 are in another group. If we fit a model with Sex (and call it the Sex_model) or SexNum (and call it the SexNum_model), we would expect the same estimates. Let’s try it.

require(coursekata) Fingers$SexNum <- as.numeric(Fingers$Sex) # fit a model of Thumb length based on Sex Sex_model <- lm() # fit a model of Thumb length based on SexNum SexNum_model <- lm() # this prints the parameter estimates from the two models Sex_model SexNum_model # fit a model of Thumb length based on Sex Sex_model <- lm(Thumb ~ Sex, data=Fingers) # fit a model of Thumb length based on SexNum SexNum_model <- lm(Thumb ~ SexNum, data=Fingers) # this prints the parameter estimates from the two models Sex_model SexNum_model ex() %>% { check_object(., "Sex_model") %>% check_equal() check_object(., "SexNum_model") %>% check_equal() check_output_expr(., "Sex_model SexNum_model") }
Call:
lm(formula = Thumb ~ Sex, data = Fingers)

Coefficients:
(Intercept)      Sexmale
     58.256        6.447
Call:
lm(formula = Thumb ~ SexNum, data = Fingers)

Coefficients:
(Intercept)       SexNum
     51.809        6.447

Because Sex is a factor (i.e., a categorical variable), lm() fits a group model. But for SexNum, lm() thinks the 1 or 2 coding refers to a quantitative variable. Because we did not tell R to treat SexNum as a factor, it fits a regression line instead of a two-group model. If it does that, the meaning of the estimates will not be what you expect for the group model.

The \(b_1\) estimate will be the same as in the two-group model; because it represents the adjustment in thumb length for a one unit change in \(X_i\). For Sex, a 1-unit change is to go from not male (\(X_i=0\)) to male (\(X_i=1\)). For \(SexNum\), a 1-unit change similarly goes from not male (\(X_i=1\)) to male (\(X_i=2\)).

\(b_1\) of the \(Sex\) model,
a group model
\(b_1\) of the \(SexNum\) model,
an accidental regression model

On the left, a representation of b-sub-one of the Sex model, a group model, as a jitter plot of Thumb predicted by Sex (female and male), with the model overlaid as horizontal lines at the mean of each group. The horizontal distance between each group is labeled as the one unit change in Sex, and the vertical distance between each group mean is labeled to say that we adjust predicted Thumb by 6.45.

On the right, a representation of b-sub-one of the SexNum model, an accidental regression model, as a jitter plot of Thumb predicted by Sex (female and male), with the model overlaid as a regression line running through the mean of each group. The horizontal distance between each group is labeled as the one unit change in SexNum, and the vertical distance between each group mean is labeled to say that we adjust predicted Thumb by 6.45.


But the \(b_0\) estimate will be different in the SexNum model, where it represents the y-intercept of the regression line, or the predicted thumb length when \(X_i\) equals 0. This makes no sense, however, when there are only two groups and they are coded 1 and 2. This is an accidental regression model.

\(b_0\) of the \(Sex\) model \(b_0\) of the \(SexNum\) model

On the left, a representation of b-sub-zero of the Sex model as a jitter plot of Thumb predicted by Sex (female and male), with the model overlaid as horizontal lines at the mean of each group. The line for the mean of the female group is labeled to say when Sex equals zero, predicted Thumb equals 58.26.

On the right, a representation of b-sub-zero of the SexNum model as a jitter plot of Thumb predicted by Sex (female and male), with the model overlaid as a regression line running through the mean of each group. The point of the regression line nearest to the y-axis, where SexNum equals zero, is labeled to say when SexNum equals zero, predicted Thumb equals 51.81.


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