[R] formula question

[Erin Hodgess]

Dear R People:

Here is a small data frame and two particular formulas:
> test.df
            y  x
1  -0.9261650  1
2   1.5702700  2
3   0.1673920  3
4   0.7893085  4
5   0.3576875  5
6  -1.4620915  6
7  -0.5506215  7
8  -0.3480292  8
9  -1.2344036  9
10  0.8502660 10
> summary(lm(exp(y)~x))

Call:
lm(formula = exp(y) ~ x)

Residuals:
    Min      1Q  Median      3Q     Max
-1.6360 -0.6435 -0.4722  0.4215  2.9127

Coefficients:
            Estimate Std. Error t value Pr(>|t|)
(Intercept)   2.1689     0.9782   2.217   0.0574 .
x            -0.1368     0.1577  -0.868   0.4108
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 1.432 on 8 degrees of freedom
Multiple R-squared: 0.08604,    Adjusted R-squared: -0.0282
F-statistic: 0.7532 on 1 and 8 DF,  p-value: 0.4108

> summary(lm(I(y^2)~x))

Call:
lm(formula = I(y^2) ~ x)

Residuals:
    Min      1Q  Median      3Q     Max
-0.9584 -0.6387 -0.2651  0.5754  1.4412

Coefficients:
            Estimate Std. Error t value Pr(>|t|)
(Intercept)  1.10084    0.62428   1.763    0.116
x           -0.03813    0.10061  -0.379    0.715

Residual standard error: 0.9138 on 8 degrees of freedom
Multiple R-squared: 0.01764,    Adjusted R-squared: -0.1052
F-statistic: 0.1436 on 1 and 8 DF,  p-value: 0.7146

>

These both work just fine.

My question is:  when do you know to use I() and just the function of
the variable, please?

thanks in advance,
Sincerely,
Erin
PS Happy St Pat's Day!


-- 
Erin Hodgess
Associate Professor
Department of Computer and Mathematical Sciences
University of Houston - Downtown
mailto: erinm.hodgess at gmail.com