Confidence Intervals for Two Samples

Return to the Main Math 160 Topics page Revised November, 2012
Some images on this page have been generated via AsciiMathML.js.
For more information see: www.chapman.edu/~jipsen/asciimath.html.

This page will examine and step through multiple solutions for two sample confidence itnervals using the TI-83/84 calculators.

We have a number of different instances to examine:

Find the confidence interval for the difference of the means given the actual data values for the two samples

We want to construct a 99.5% confidence interval for the difference between two sample means from independent samples. We are given the following sample statistics:
 sizemeanstnd. dev.
sample 14257.903.60
sample 24763.603.40
Figure 1
We are going to create a confidence interval given by

`barx_1 - barx_2 +- t_(alpha/2)*sqrt((s_1^2)/(n_1) + (s_2^2)/(n_2))` using the smaller of `(n_1-1)` and `(n_2-1)` as the degrees of freedom. We use this as the "simple method" for determining the number of degrees of freedom. It is certainly the case that the more complex method, shown later, will give a "higher" degrees of freedom. However, higher degrees of freedom translate into smaller t-values which in turn yield smaller margins of error which result in narrower confidence intervals. By using the "simple method" our final result will be a broader confidence interval, one that will most certainly cover the more compact one found later.

Our first step is to determine the t-value to use for a 99.5% confidence interval. On a TI-84 (with the newer operating system) we have the invT( option in the DIST menu ( ).

Figure 2
The command that we want is invT(.9975,41) where the .9975 comes from the fact that we want 99.5% inside the confidence interval, leaving 0.5% outside, and that has to be split, half above and have below. Thus, there will only be 0.25% to the right. The invT( command returns the t-value having the specified area (for us, .9975) for all values to the left of the point, and for the given degrees of freedom. The calculator produces the t-value: 2.96696131.

For those having a calculator without the invT( option, we do have the INVT program. This will give approximately the same answer. In Figure 2 we start the program, giving the required values.

Figure 3
Figure 3 shows the rest of the program output. The value produced is 2.966928428. Not quite as good as the better and faster built-in routine, but good enough for our purposes. Note that the program also give us the fact that the value it produced has .9974997809 of the area to its left, not the asked for .9975.
Figure 4
The variance of the difference of the two means is the sum of the variances of the two means. Since we do not know the variance of the populations we use the variance of the samples (which is why we used the t-values rather than use z-scores above). The desired variance is thus `s_1^2/n_1 + s_2^2/n_2`. This, of course, makes the standard deviation of the difference of the means be given by `sqrt(s_1^2/n_1 + s_2^2/n_2)`. For our problem this becomes `sqrt(3.6^2/42+3.4^2/47`, an expression given in Figure 5. Thus, for this problem the standard deviation of the sample means is .744666956.

The computation for the combined standard deviation is not terrible to remember, but it is a pain to type. One might consider writing a program such as
In that program lines 5 through 10 prompt the user for the required information. Lines 11 through 13 compute the combined variance. Line 14 computes the standard deviation. Lines 15 through 17 display the computed values. Note that the program goes on to do a bit more calculation, but we will not present that until Figure 11.
Figure 5
Figure 5 shows the start of running the program CALCVSDF, providing exactly the data from the problem we are doing.
Figure 6
Figure 6 completes the data entry.
Figure 7
Figure 7 provides the computed results. Note that the display gives both the variance and the standard deviation, the latter being exactly what we found back in Figure 4.
Figure 8
The margin of error is the product of the t-value and the standard deviation of the difference of the means. For this problem we actually type in the two values that we found first in Figure 2 and in Figure 4. [We might note that by reading through the program we can see that the standard deviation value is stored in the variable S. Therefore, we could have written the our expression as 2.966928*S.]
Figure 9
In Figure 9 we first store the margin of error in X. Then we compute the difference in the means, `x_1-x_2`, and store it in A. That means that we can get the lower end of the confidence interval via A-X.
Figure 10
Then, it is easy to get the upper end of the confidence interval from A+X. Combining these we have the confidence interval as (-7.909, -3.4906).
Figure 11
The work done so far has been based, as noted in Figure 1, on the "simple method" for determining the number of degrees of freedom that we will use. There is complex formula to derive a larger number of degrees of freedom which will result in a smaller t=value, thus closing the confidence interval a bit. This other formula is `(( (s_1^2)/(n_1) + (s_2^2)/(n_2) )^2) / ((((s_1^2)/(n_1))^2)/(n_1-1)+(((s_2^2)/(n_2))^2)/(n_2-1))`. We would not want to type that into the calculator each time we solve a problem. However, it is not too much to include it in a program. That is exactly what was done in the program given above. Lines 18 and 19 compute the numerator and the denominator of the expression. Lines 20 and 20 display the result. Thus, back in Figure 7, we were given the additional information that the degrees of freedom can be computed to be 84.5376476.

In Figure 11 we have used this strange value with the invT( command to get a new t-value, namely, 2.883070818, As expected this is less than the 2.96696131 we found using 41 degrees of freedom in Figure 2. Note that we not only generate the answer, we have also stored it in the variable T.

The computed "degrees of freedom" is "strange" in that it is not a whole number. The TI calculator does not have a problem with this but we sure are not going to find and table in a text that has t-values for 2.882575285 degrees of freedom. We take a minute here to see just how much the t-value changes with slight changes in the degrees of freedom. For example, if we round down to 84 degrees of freedom we find a value of 2.883070818. This is not much different from 2.883070818.

Figure 12
In Figure 12 we look at the same computation with 85 degrees of freedom, generating 2.8821542185, also not much different from 2.883070818.

Then, returning to use the value we stored into the varaible T in Figure 11, we walk through the same steps that we used in Figures 9 and 10 to find the margin of error, and store it in X. This new margin of error is 2.146558563, again smaller than the value we found using the simple method above.

We go on to find the lower end of the confidence interval.

Figure 13
And we use A+X to get the upper end. The new confidence interval, (-7.84656, -3.5534) is indeed narrower than the interval we found in Figure 10.
Figure 14
To this point we have done this problem by following the formula for finding the confidence interval for the difference of two means. Now we will do the same problem but we will let the calculator do all the work.

In the STAT menu, under the TESTS submenu, we find the 2-SampTInt... command. Selecting this command takes us to the input screen shown in Figure 15.

Figure 15
Figure 15 shows the screen as it was found on a particular calculator. In this case, the input screen is set to input locations of actual Data. We, however, have a problem that gives us the statistics , not the data of the experiment. We use the key to move the blinking highlight over to the Stat option and then press to move to Figure 16.
Figure 16
Now we have the input screen changed to accept statistic. We continue by entering the required values: `barx_1=57.9`, `S_(x_1)=3.6`, `n_1=42`, `barx_2=63.6`, `S_(x_2)=3.4`, and `n_2=47`.

Then use to move down for the remaining settings.

Figure 17
Make sure that we specify the given confidence level as the full 99.5%, entered as .995.

Leave the "Pooled" option on No.

Highlight the calculate option and press .

Figure 18
The result, shown in Figure 18, gives the confidence interval (the same as we found in Figures 12 and 13) and the computed degrees of freedom that we found (shown in Figure 7 but discussed in Figure 11).
Figure 18a
One of the values not shown in the output of the 2-SampTInt command is the margin of error. However, since the margin of error is just half the width of the confidence interval we could calculate it. Such a calculation is shown in Figure 18a. This should be compared to the 2.146558563 found in Figure 12. The difference is, of course, due to the use of the rounded values for the confidence interval in Figure 18.
Figure 18b
We could retrieve the more complete values for the lower and upper ends of the confidence interval by moving to the VARS menu, selecting the Statistics option, and then moving to the bottom of the TEST sub-menu.

To move to Figure 18c we press to paste the identifier upper onto the main screen.

Figure 18c
Here we see the longer version of the upper end of the confidence interval to be -3.553441437.

Figure 18c goes on to form the expression (upper-lower)/2 to generate the same value that we found back in Figure 12.

Typing the expression (upper-lower)/2 takes between 20 and 45 keystrokes, depending on how you maneuver around the calculator screens. Even at 20 this is an excessive process. I(f we need to do this many times then this would be a perfect case for creating a tiny program to just do and display the computation for us. We will do that.

Figure 18d
gets us to Figure 18d. From there we use to move to Figure 18e.
Figure 18e
Here we give the program a name; our name will be MOE. The calculator starts Figure 18e in alphabetic mode, so press to generate MOE Press to move to Figure 18f.
Figure 18f
Here we are ready to write the lines of the program. The only line we want to construct is Disp (upper-lower)/2. We use to move to Figure 18g where we can find the Disp command.
Figure 18g
Highlight the Disp command and press .
Figure 18h
We have pasted the Disp command into the program.
Figure 18i
Then we go through the 20 to 45 keystrokes to generate the rest of the command, ending with a .

The program is complete. Press to get out of the program editor and return to the main screen.

Figure 18j
On the main screen we have recalled the program and run it. The output is what we should expect.

One small warning is appropriate here. The program that we just wrote uses values that have been stored into upper and lower. You should use this program immediately after you have constructed a confidence interval. The program will simply use the values that it finds in upper and lower. The program will not recall some much earlier computation of these values.


Find the confidence interval for the difference of the means given the actual data values for the two samples

Based upon our samples we want to construct 99.0% confidence intervals for the difference of the two means.

Figure 19
In this case we are given the actual data. One approach, and the first one used here, is to just get the statistics that we need and then follow the same path that we took in the first example.

To that end, we generate the data. Note that the generation program, GNRND4, always generates this data in L1. Therefore, in order to keep the calculator looking like the problem that we have been given, we will actually generate the second set of data first. Then, we will store that data into L2 before we generate the first set of data. As a result, when we finally get to Figure 33 we will have the first set of data in L1 and the second set in L2.

Figure 20
Once the data has been generated, we can do a 1-Var Stats on it.
Figure 21
The three values we need are given here, the mean, the standard deviation, and the sample size. Remember that these are for the second set of data. Therefore we have `x_2=56.56818182`, `S_(x_2)=5.263952566` and `n_2=44`.

Note that at the bottom of Figure 21 we have copied L1 to L2.

Figure 22
Now generate the first data set. [This will overwrite the data that was in L1.]
Figure 23
Get the statistics for the first data set.
Figure 24
Therefore we have `x_1=55.4170218`, `S_(x_2)=5.050928419` and `n_2=47`.
Figure 25
Recalling that we can compute the combined variance and standard deviation, not to mention the complex number of degrees of freedom, by using the program shown above (between Figures 4 and 5), we run that program. Figure 25 shows the start of the program and the first values that we have entered.
Figure 26
Figure 26 shows the rest of the data input for the program.
Figure 27
Figrue 27 gives us the combined standard deviation, namely, 1.082848272 and the complex number of degrees of freedom, namely, 87.97533872.
Figure 28
Again, following our original path in example 1 above, we find the required t-value by using invT, giving it the .995 value (because we split the 1% between the top and the bottom) and the complex number of degrees of freedom. That t-value is computed to be 2.632874324.

We then use that answer times the combined standard deviation to get the margin of error which we store in X as 2.851003412.

Finally, in Figure 28, we compute the difference of the means and store that in A.

Figure 29
Having done all that, we can get the lower and upper ends of the confidence interval via A-X and A+X, respectively.
Figure 30
Alternatively, as we did for the first example, we could just move to the STAT menu, TEST sub-menu, and choose the 2-SampTInt... option. This takes us back to our input screen, shown in Figure 30 after we have entered the values for this problem.
Figure 31
Figure 31 shows the rest of the data input, including setting the confidence level at .99.

Once the values have been entered we highlight the Calculate option and press .

Figure 32
The results are given in Figure 32.

At this point we have simply followed the path we laid out in the first example above. However, the TI calculators give us another option.

Figure 33
We return to the 2-SampTInt screen and change the option back to DATA.

Now the calculator is looking for the raw data. We tell the calculator that the first list of data is in L1 and the second list is in L2. These lists contain the data values. There are no associated frequency lists. Therefore, we leave the Freq1: and Freq2: settings as 1.

Set the confidence level at .99.

Figure 34
Move to the Calculate option. Press to have the calculator do all the work.
Figure 35
The results are shown in Figure 35.

Find the confidence interval for the difference of proportions given the statistics of sample size and proportions

We are given the following statistics for two independent random samples of a large population:

 Sample
Size
Number of
desired Items
Sample 17543
Sample 19462
We want to construct a 95.0% confidence interval for the difference, `hatp_1-hatp_2`, for the two proportions of the desired items.
Figure 36
The formula that we want is that the confidence interval is given by `hatp_1 - hatp_2 +- (z_(alpha/2))* sqrt(( hatp_1*(1-hatp_1) )/(n_1) + ( hatp_2*(1-hatp_2) )/(n_2) )` Therefore, we compute `hatp_1` as 43/75 and store it in A. Then we compute `hatp_2` as 62/94 and store it in B. At that point it is a small taks to compute the variance of the difference of the proportions as A*(1-A)/75+B*(1-B)/94 and store that in C.
Figure 37
Then, taking the square root of C we get the standard error of the difference of the proportions and store that in D.
Figure 38
We remember that we are using the normal distribution for the difference of the proportions. Therefore we need to get our z-score. From the DIST menu we select invNorm(.
Figure 39
We complete the command by asking for the .975 z-score. The result is 1.959963986.

Multiply that answer by the standard error which we stored in D and we get the margin of error, .1473276438, which we store in X.

Find the difference of the proportions and store that in F.

Form the command F-X to find the lower end of the confidence interval.

Figure 40
The command F+X will give us the upper end of the confidence interval.
Figure 41
Now that we have walked through the computation, we return to let the calculator do all the work.

In the STAT menu, the TESTS sub-menu, we find the 2-PropZInt... command. We select that command and press .

Figure 42
This brings up the data input screen. We give that screen the values from the problem, including the confidence level. Then highlight the Calculate option and press .
Figure 43
The calculator does all the work and displays both the confidence interval and the individual proportions.

Find the confidence interval for the difference of proportions given the actual data values for the two samples


We are interested in the difference in the proportions of characteristic 3 in the underlying popualation. We want to construct a 95.0% confidence interval for `hatp_1-hatp_2`.
Figure 44
The first thing to do is to generate the data. Here we generate Data Set 3.
Figure 45
Notice that the program reports that it will generate 88 values.

Our particular concern in this problem is the proportion of 3's in those 88 values. Once the data is generated we need a way to find the number of 3's in the list.

Figure 46
One of the most straight forward methods is to just create a histogram of the values. In Figure 47 we have moved to the Plot1 screen and made sure that the plot is ON, that we will be doing a histogram, and that the data for the histogram will come from L1.
Figure 47
We then move to the ZOOM menu and use the ZoomStat option.
Figure 48
Once the histogram is displayed we move to use the TRACE feature and then move the highlight onto what we know is the bar for the 3's. From Figure 48, at the bottom, we see that there are 30 such values in our data.
Figure 49
An alternative approach is to use the COLLATE2 program. Please note that the version shown here is version 2.1. This version includes a prompt to let the program complete without displaying all of the computed statistics.

We tell the program that the data is in L1.

Figure 50
The program then tells us that there are 88 items that were found. This version then asks if we want all of the statistics. Our response is 0 to indicate "no".
Figure 51
After the program has completed we move to the Stat Editor. Here we find that ITEM 3 has been found 30 times.

Either way approach the problem we now know that the first list has 30 items that are 3 out of a total of 88 items.

Figure 52
Now we can generate the other set of data.
Figure 53
Again, we need to find the number of 3's in this list. Figure 53 uses the histogram method to do this. We can see that there are 23 instances of 3 in this data set.
Figure 54
Of course, we could go back through all the steps that we took in Figures 36 through 40 to compute the confidence interval. However, using the 2-PropZInt... command will take less time.

Here we have used that command to open the input form. Then we go ahead and supply all of the values, including the confidence level. When all the values are in place we highlight the Calculate option and press .

Figure 55
The result is the confidence interval given in Figure 55.

Some note should be taken that by using this method we do not get to see any of the intermediary values such as the standard deviation of the proportion difference and the margin of error. We can take care of the latter by running the program we saw above, MOE.

Figure 55a
This gives us the margin of error based upon the values of lower and higher that we just computed.

As for the other values, those computations can easily be placed into a program. Let us look at the listing of the program CALCPSD given below.



This program merely captures the computations that we did back in Figures 36 and 37.

Figure 56
Figure 56 gives the start of a run of the program. Note that we supply the sample size as 88 and the proportion (which know in decimal format from Figure 55) we merely enter as 30/88, letting the calculator recompute that value.
Figure 57
Figure 57 completes the data entry for the program.
Figure 58
Figure 58 shows the output.
Figure 59
Figure 59 merely demonstrates that we could have done the calculations by hand. In this case we use the confidence interval endpoints to re-compute the margin of error.

Find the confidence interval for the difference of the means for paired data values given the actual data values for the two samples

We want to construct a 95.0% confidence interval for the difference between the paired measures.

Figure 60
As usual, we can start by generating the two lists of paired values. Note that for paired values the GNRND4 program produces lists in L1 and L2.
Figure 61
The whole idea of the paired values is to derive the difference within each pair. We do this and store the result in L3.

Then we can look at the 1-Var Stats for L3.

Figure 62
This particular problem then resolves to finding the 95% confidence interval for the mean of the difference. That will be `barx_d +- t_(alpha/2)(s_d)/sqrt(n)`. We use the values given here to do those computations.
Figure 63
To find `s_d/sqrt(n)`, rather than retype the value for `s_d` we move to the VARS, Statistics menu and select `S_x`.
Figure 64
Using that we can compute `S_x/sqrt(25)` and store it in S.

Then find the required T-value and store it in T.

Next, find the product S*T, the margin of error, and store it in X.

Figure 65
We return to the VARS, Statistics menu to retrieve `barx`.

Finally, we form `barx - X` and `barx + X` to get the limits on the confidence interval.

Figure 66
Of course, since this is just getting the confidence interval for the difference of the paired values, we could have let the calculator do all the work. We just need to move to the STAT menu, the TESTS sub-menu, and move down to the TInterval option.
Figure 67
This brings up the data input screen. We actually have the data stored in L3. Making sure that we tell the calculator that the data is in L3 and that we want a .95 confidence level, we highlight the Calculate option and press .
Figure 68
The calcualtor does all the work and produces the confidence interval.

©Roger M. Palay
Saline, MI 48176
November, 2012