# Preliminaries

You use ttestGC() for inferential procedures regarding:

• one population mean $$\mu$$;
• the difference of two population means, $$\mu_1 - \mu_2$$;
• the mean $$\mu_d$$ of paired differences in a population.

The function comes from the tigerstats package, so make sure that tigerstats is loaded:

require(tigerstats)

# One Mean, From Data Frame (Read This!!)

• levels of confidence,
• types of Alternative Hypothesis,
• graphs of the P-value, and
• an option to limit output to the console

in ways that apply to all uses of ttestGC().

## Confidence Interval Only

In the m111survey data from the tigerstats package, suppose you want a 95%-confidence interval for:

$$\mu =$$ the mean fastest speed ever driven, for all GC students.

Use the function:

ttestGC(~fastest,data=m111survey)
##
##
## Inferential Procedures for One Mean mu:
##
##
## Descriptive Results:
##
##  variable  mean    sd  n
##   fastest 105.9 20.88 71
##
##
## Inferential Results:
##
## Estimate of mu:   105.9
## SE(x.bar):    2.478
##
## 95% Confidence Interval for mu:
##
##           lower.bound         upper.bound
##           100.959833          110.842984

To get any other level of confidence, you need to use the conf.level argument, indicating your desired level of confidence in decimal (not percentage) form.

For example, for a 90%-confidence interval for $$\mu$$, use

ttestGC(~fastest,data=m111survey,conf.level=0.90)
##
##
## Inferential Procedures for One Mean mu:
##
##
## Descriptive Results:
##
##  variable  mean    sd  n
##   fastest 105.9 20.88 71
##
##
## Inferential Results:
##
## Estimate of mu:   105.9
## SE(x.bar):    2.478
##
## 90% Confidence Interval for mu:
##
##           lower.bound         upper.bound
##           101.771329          110.031488

## Interval and Test

If you want a test of significance as well as the confidence interval, then

• use the mu argument to set what the Null Hypothesis thinks that the value of $$\mu$$ is;
• use the alternative argument to specify the Alternative Hypothesis.

For example, if the hypotheses are:

$$H_0: \mu = 100$$

$$H_a: \mu > 100$$

Then use:

ttestGC(~fastest,data=m111survey,mu=100,alternative="greater")
##
##
## Inferential Procedures for One Mean mu:
##
##
## Descriptive Results:
##
##  variable  mean    sd  n
##   fastest 105.9 20.88 71
##
##
## Inferential Results:
##
## Estimate of mu:   105.9
## SE(x.bar):    2.478
##
## 95% Confidence Interval for mu:
##
##           lower.bound         upper.bound
##           101.771329          Inf
##
## Test of Significance:
##
##  H_0:  mu = 100
##  H_a:  mu > 100
##
##  Test Statistic:     t = 2.382
##  Degrees of Freedom:   70
##  P-value:        P = 0.009974

If the hypotheses are:

$$H_0: \mu = 100$$

$$H_a: \mu < 100$$

Then use:

ttestGC(~fastest,data=m111survey,mu=100,alternative="less")
##
##
## Inferential Procedures for One Mean mu:
##
##
## Descriptive Results:
##
##  variable  mean    sd  n
##   fastest 105.9 20.88 71
##
##
## Inferential Results:
##
## Estimate of mu:   105.9
## SE(x.bar):    2.478
##
## 95% Confidence Interval for mu:
##
##           lower.bound         upper.bound
##           -Inf                110.031488
##
## Test of Significance:
##
##  H_0:  mu = 100
##  H_a:  mu < 100
##
##  Test Statistic:     t = 2.382
##  Degrees of Freedom:   70
##  P-value:        P = 0.99

If the hypotheses are:

$$H_0: \mu = 100$$

$$H_a: \mu \neq 100$$

Then use:

ttestGC(~fastest,data=m111survey,mu=100,
alternative="two.sided")
##
##
## Inferential Procedures for One Mean mu:
##
##
## Descriptive Results:
##
##  variable  mean    sd  n
##   fastest 105.9 20.88 71
##
##
## Inferential Results:
##
## Estimate of mu:   105.9
## SE(x.bar):    2.478
##
## 95% Confidence Interval for mu:
##
##           lower.bound         upper.bound
##           100.959833          110.842984
##
## Test of Significance:
##
##  H_0:  mu = 100
##  H_a:  mu != 100
##
##  Test Statistic:     t = 2.382
##  Degrees of Freedom:   70
##  P-value:        P = 0.01995

But note that the default value of alternative is “two.sided”, so if you want you could just leave it out and still get a two-sided test:

ttestGC(~fastest,data=m111survey,mu=100)

## Graph of the P-Value

Anytime you want, you can get a graph of the $$P$$-value for your test, simply by setting the argument graph to TRUE:

ttestGC(~fastest,data=m111survey,mu=100,
alternative="two.sided",graph=TRUE)
##
##
## Inferential Procedures for One Mean mu:
##
##
## Descriptive Results:
##
##  variable  mean    sd  n
##   fastest 105.9 20.88 71
##
##
## Inferential Results:
##
## Estimate of mu:   105.9
## SE(x.bar):    2.478
##
## 95% Confidence Interval for mu:
##
##           lower.bound         upper.bound
##           100.959833          110.842984
##
## Test of Significance:
##
##  H_0:  mu = 100
##  H_a:  mu != 100
##
##  Test Statistic:     t = 2.382
##  Degrees of Freedom:   70
##  P-value:        P = 0.01995 ## Limiting Output

Sometimes you don’t need R to print so much information to the console. If you want only the basics (such as a confidence interval, the test statistic and $$P$$-value), then set the verbose argument to FALSE:

ttestGC(~fastest,data=m111survey,mu=100,
alternative="two.sided",verbose=FALSE)
##
##
## Inferential Procedures for One Mean mu:
## 95% Confidence Interval for mu:
##
##           lower.bound         upper.bound
##           100.959833          110.842984
##
##  Test Statistic:     t = 2.382
##  Degrees of Freedom:   70
##  P-value:        P = 0.01995

# One Mean, From Summary Data

## Confidence Interval Only

Say that you have taken a simple random sample from some large population, and:

• the sample mean is $$\bar{x}=30$$,
• the sample standard deviation is $$s=4$$,
• the sample size was $$n=40$$.

You don’t have the raw data present in a data frame, but you have enough summary data to use ttestGC(). You just have to set some new arguments:

• set mean to the sample mean;
• set sd to the sample standard deviation;
• set n to the sample size.

So if you only want a 95%-confidence interval for $$\mu$$. use:

ttestGC(mean=30,sd=4,n=40)
##
##
## Inferential Procedures for One Mean mu:
##
##
## Descriptive Results:
##
##  mean sd  n
##    30  4 40
##
##
## Inferential Results:
##
## Estimate of mu:   30
## SE(x.bar):    0.6325
##
## 95% Confidence Interval for mu:
##
##           lower.bound         upper.bound
##           28.720738           31.279262

## Interval and Test

If you also want to do a test of significance, again specify mu and alternative. For example, to test the hypotheses

$$H_0: \mu = 32$$

$$H_a: \mu < 32$$

use:

ttestGC(mean=30,sd=4,n=40,
mu=32,alternative="less")
##
##
## Inferential Procedures for One Mean mu:
##
##
## Descriptive Results:
##
##  mean sd  n
##    30  4 40
##
##
## Inferential Results:
##
## Estimate of mu:   30
## SE(x.bar):    0.6325
##
## 95% Confidence Interval for mu:
##
##           lower.bound         upper.bound
##           -Inf                31.065609
##
## Test of Significance:
##
##  H_0:  mu = 32
##  H_a:  mu < 32
##
##  Test Statistic:     t = -3.162
##  Degrees of Freedom:   39
##  P-value:        P = 0.001514

# Difference of Two Means, Data Frame

## Confidence Interval Only

Suppose

$$\mu_1 =$$ mean fastest speed ever driven, by all GC females

$$\mu_2 =$$ mean fastest speed ever driven, by all GC males

If you desire, say, an 85%-confidence interval for $$\mu_1 - \mu_2$$, then use:

ttestGC(fastest~sex,data=m111survey,
conf.level=0.85)
##
##
## Inferential Procedures for the Difference of Two Means mu1-mu2:
##  (Welch's Approximation Used for Degrees of Freedom)
##   fastest grouped by sex
##
##
## Descriptive Results:
##
##   group  mean    sd  n
##  female 100.0 17.61 40
##    male 113.5 22.57 31
##
##
## Inferential Results:
##
## Estimate of mu1-mu2:  -13.4
## SE(x1.bar - x2.bar):  4.918
##
## 85% Confidence Interval for mu1-mu2:
##
##           lower.bound         upper.bound
##           -20.579953          -6.223273

## Interval and Test

If you want a 95%-confidence interval for $$\mu_1 - \mu_2$$ and you would like to test the hypotheses:

$$H_0: \mu_1 - \mu_2 = 0$$

$$H_a: \mu_1 - \mu_2 \neq 0$$

then use:

ttestGC(fastest~sex,data=m111survey,
mu=0)
##
##
## Inferential Procedures for the Difference of Two Means mu1-mu2:
##  (Welch's Approximation Used for Degrees of Freedom)
##   fastest grouped by sex
##
##
## Descriptive Results:
##
##   group  mean    sd  n
##  female 100.0 17.61 40
##    male 113.5 22.57 31
##
##
## Inferential Results:
##
## Estimate of mu1-mu2:  -13.4
## SE(x1.bar - x2.bar):  4.918
##
## 95% Confidence Interval for mu1-mu2:
##
##           lower.bound         upper.bound
##           -23.254640          -3.548586
##
## Test of Significance:
##
##  H_0:  mu1-mu2 = 0
##  H_a:  mu1-mu2 != 0
##
##  Test Statistic:     t = -2.725
##  Degrees of Freedom:   55.49
##  P-value:        P = 0.008579

Notice that this time:

• you specified the Null’s belief about the value of $$\mu_1-\mu_2$$ using the mu argument;
• you did not have to set conf.level to 0.95 (the default value of the argument is 0.95 already);
• you did not have to specify the two-sided alternative (the default value of alternative is already “two.sided”).

## Order of the Groups

Suppose that in the previous situation you had defined:

$$\mu_1 =$$ mean fastest speed ever driven, by all GC males

$$\mu_2 =$$ mean fastest speed ever driven, by all GC females

Then for you, the first population is all GC males and the second population is all GC females. In order to guarantee that R abides by your choice, use the argument first:

ttestGC(fastest~sex,data=m111survey,
mu=0,first="male")
##
##
## Inferential Procedures for the Difference of Two Means mu1-mu2:
##  (Welch's Approximation Used for Degrees of Freedom)
##   fastest grouped by sex
##
##
## Descriptive Results:
##
##   group  mean    sd  n
##    male 113.5 22.57 31
##  female 100.0 17.61 40
##
##
## Inferential Results:
##
## Estimate of mu1-mu2:  13.4
## SE(x1.bar - x2.bar):  4.918
##
## 95% Confidence Interval for mu1-mu2:
##
##           lower.bound         upper.bound
##           3.548586            23.254640
##
## Test of Significance:
##
##  H_0:  mu1-mu2 = 0
##  H_a:  mu1-mu2 != 0
##
##  Test Statistic:     t = 2.725
##  Degrees of Freedom:   55.49
##  P-value:        P = 0.008579

# Difference of Two Means, Summary Data

## Confidence Interval Only

Suppose that you have taken two independent samples from two populations (or performed a completely randomized experiment with two treatment groups), and you have the following summary data:

Group $$\bar{x}$$ $$s$$ $$n$$
group one 32 4.2 33
group two 30 5.1 42

You need to provide the summary data to the arguments mean, sd and n, as lists using the c() function. In each list, data from the first group should come first.

For a 95%-confidence interval for $$\mu_1 - \mu_2$$, use:

ttestGC(mean=c(32,30),sd=c(4.2,5.1),n=c(33,42))
##
##
## Inferential Procedures for the Difference of Two Means mu1-mu2:
##  (Welch's Approximation Used for Degrees of Freedom)
##  Results from summary data.
##
##
## Descriptive Results:
##
##    group mean  sd  n
##  Group 1   32 4.2 33
##  Group 2   30 5.1 42
##
##
## Inferential Results:
##
## Estimate of mu1-mu2:  2
## SE(x1.bar - x2.bar):  1.074
##
## 95% Confidence Interval for mu1-mu2:
##
##           lower.bound         upper.bound
##           -0.140898           4.140898

## Interval and Test

Suppose that you want a 90%-confidence interval for $$\mu_1 - \mu_2$$ and that you would like to test the hypotheses:

$$H_0: \mu_1 - \mu_2 = 0$$

$$H_a: \mu_1 - \mu_2 > 0$$

Suppose also that want a graph of the $$P$$-value. Then use:

ttestGC(mean=c(32,30),sd=c(4.2,5.1),n=c(33,42),
mu=0,alternative="greater",
conf.level=0.90,graph=TRUE)
##
##
## Inferential Procedures for the Difference of Two Means mu1-mu2:
##  (Welch's Approximation Used for Degrees of Freedom)
##  Results from summary data.
##
##
## Descriptive Results:
##
##    group mean  sd  n
##  Group 1   32 4.2 33
##  Group 2   30 5.1 42
##
##
## Inferential Results:
##
## Estimate of mu1-mu2:  2
## SE(x1.bar - x2.bar):  1.074
##
## 90% Confidence Interval for mu1-mu2:
##
##           lower.bound         upper.bound
##           0.610797            Inf
##
## Test of Significance:
##
##  H_0:  mu1-mu2 = 0
##  H_a:  mu1-mu2 > 0
##
##  Test Statistic:     t = 1.862
##  Degrees of Freedom:   72.82
##  P-value:        P = 0.03333 # Mean of Differences

## Both Variables in the Data Frame

### Confidence Interval Only

Suppose that

$$\mu_d =$$ mean difference (ideal height minus actual height) for all Georgetown College student.

Both of the relevant variables—ideal_ht and height—are present in the m111survey data frame.

If you want a 95%-confidence interval for $$\mu_d$$, then use:

ttestGC(~ideal_ht - height,data=m111survey)
##
##
## Inferential Procedures for the Difference of Means mu-d:
##   ideal_ht minus height
##
##
## Descriptive Results:
##
##         Difference mean.difference sd.difference  n
##  ideal_ht - height           1.946         3.206 69
##
##
## Inferential Results:
##
## Estimate of mu-d:     1.946
## SE(d.bar):    0.3859
##
## 95% Confidence Interval for mu-d:
##
##           lower.bound         upper.bound
##           1.175528            2.715776

Note how the “~” character signals the presence of a formula.

### Interval and Test

In order to test:

$$H_0: \mu_d = 0$$

$$H_a: \mu_d > 0$$

use:

ttestGC(~ideal_ht - height,data=m111survey,
mu=0,alternative="greater")
##
##
## Inferential Procedures for the Difference of Means mu-d:
##   ideal_ht minus height
##
##
## Descriptive Results:
##
##         Difference mean.difference sd.difference  n
##  ideal_ht - height           1.946         3.206 69
##
##
## Inferential Results:
##
## Estimate of mu-d:     1.946
## SE(d.bar):    0.3859
##
## 95% Confidence Interval for mu-d:
##
##           lower.bound         upper.bound
##           1.302075            Inf
##
## Test of Significance:
##
##  H_0:  mu-d = 0
##  H_a:  mu-d > 0
##
##  Test Statistic:     t = 5.041
##  Degrees of Freedom:   68
##  P-value:        P = 1.826e-06

## The Difference is in the Data Frame

Sometimes the difference of the two relevant numerical variables is included in the data frame as another variable: such is the case for m111survey, where the difference is recorded as the variable diff.ideal.act.

If you would like to use this difference, then work just as if you were studying one population mean $$\mu$$.

### Confidence Interval Only

Thus, for a 95%-confidence interval for $$\mu_d$$ we could also have done:

ttestGC(~diff.ideal.act.,data=m111survey)
##
##
## Inferential Procedures for One Mean mu:
##
##
## Descriptive Results:
##
##         variable  mean    sd  n
##  diff.ideal.act. 1.946 3.206 69
##
##
## Inferential Results:
##
## Estimate of mu:   1.946
## SE(x.bar):    0.3859
##
## 95% Confidence Interval for mu:
##
##           lower.bound         upper.bound
##           1.175528            2.715776

### Interval and Test

For a test of

$$H_0: \mu_d = 0$$

$$H_a: \mu_d > 0$$

we could have done:

ttestGC(~diff.ideal.act.,data=m111survey,
mu=0,alternative="greater")
##
##
## Inferential Procedures for One Mean mu:
##
##
## Descriptive Results:
##
##         variable  mean    sd  n
##  diff.ideal.act. 1.946 3.206 69
##
##
## Inferential Results:
##
## Estimate of mu:   1.946
## SE(x.bar):    0.3859
##
## 95% Confidence Interval for mu:
##
##           lower.bound         upper.bound
##           1.302075            Inf
##
## Test of Significance:
##
##  H_0:  mu = 0
##  H_a:  mu > 0
##
##  Test Statistic:     t = 5.041
##  Degrees of Freedom:   68
##  P-value:        P = 1.826e-06

Note that in its statement of hypotheses, R identifies the parameter of interest as $$\mu$$ rather than as $$\mu_d$$. It had no way of knowing that diff.ideal.act. recorded the difference of a pair of measurements.