Extracting Observations

Comparing Analyses

Extracting Observations

You can compare analyses made with different observations by extracting a subset, that is, by creating a new data set that contains a subset of observations from the original data set. Then you can request separate analyses for each data set. Consider the MINING data. This data set contains results of an experiment to examine drilling times (DRILTIME) for different drilling methods (METHOD). As it turned out, the experimenters encountered difficulties due to changing rock types after a depth of about 200 feet. It might be worthwhile to compare the distribution of DRILTIME for depths greater than 200 feet to the distribution of DRILTIME for the entire data set. To compare the two distributions, you need to select the observations where DEPTH is greater than 200 feet and extract them into a new data window.

Open the MINING data set.

Figure 21.2: MINING Data

Choose Edit:Observations:Find.

[menu]
Figure 21.3: Finding Observations

This displays the Find Observations dialog.

Figure 21.4: Find Observations Dialog

Select > in the Test list and 200 in the Value list.

Figure 21.5: Finding DEPTH > 200

Click the OK button.

This selects all observations where DEPTH is greater than 200 feet. To see the selected observations, either choose Find Next from the data pop-up menu or scroll down using the vertical scroll bar on the right (as indicated by the arrow in the figure).

Figure 21.6: Observations where DEPTH > 200

Choose Extract from the data pop-up menu.

A new data set containing observations where DEPTH is greater than 200 feet appears, as shown in Figure 21.7. The new data window is named automatically by adding a subscript to the original name. You may have to scroll to the top of the data window to duplicate the next figure.

Figure 21.7: MINING1 Data

Now create distribution analyses for both data sets.

Select DRILTIME in the MINING data window.

Choose Analyze:Distribution (Y).

A distribution analysis using all the observations appears on your display.

Select DRILTIME in the MINING1 data window.

Choose Analyze:Distribution (Y).

A distribution analysis using the subset of observations appears on your display.

Move the two analysis windows side-by-side to compare the distributions.

Figure 21.8: Comparing Two Distribution Analyses

The mean drilling time at depths greater than 200 feet was 9.9601, while the mean overall was only 8.8589. The drills may have found harder rock at greater depths. You may want to create an additional analysis to compare depths greater than 200 feet with depths less than or equal to 200 feet.

Choose File:End to delete MINING1 and the two analysis windows.

Note Sometimes you will want to compare analyses that use different subsets of observations based on the values of some variable. If this is the case, you can assign the variable the Group role, as described in Chapter 22, "Analyzing by Groups."

Related
Reading Distributions, Chapter 38.

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