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The FREQ Procedure


Inputting Frequency Counts
PROC FREQ can use either raw data or cell count data to produce frequency and crosstabulation tables. Raw data, also known as case-record data, report the data as one record for each subject or sample member. Cell count data report the data in tabular form. A table lists all possible combinations of the data values along with the frequency counts. This way of presenting data often appears in published results.

The following DATA step statements store raw data in a SAS data set:

data raw;
   input subject $ R C @@;
01 1 1  02 1 1  03 1 1  04 1 1  05 1 1
06 1 2  07 1 2  08 1 2  09 2 1  10 2 1
11 2 1  12 2 1  13 2 2  14 2 2  15 2 2

You can store the same data as cell counts using the following DATA step statements:

data counts;
   input R C CellCount @@;
1 1 5   1 2 3
2 1 4   2 2 3
The variable R contains the values for the rows and the variable C contains the values for the columns. The variable CellCount contains the cell count for each row and column combination.

Both the RAW data set and COUNTS data set produce identical frequency counts, two-way tables, and statistics. With the COUNTS data set, you must use a WEIGHT statement to specify that CellCount contains cell counts. For example, to create a two-way crosstabulation table submit the following statements:

proc freq data=counts;
   weight CellCount;
   tables R*C;

Grouping with Formats
PROC FREQ groups a variable's values according to its formatted values. If you assign a format to a variable with a FORMAT statement, PROC FREQ formats the variable values before dividing observations into the levels of a frequency or crosstabulation table.

For example, suppose that a variable X has the values 1.1, 1.4, 1.7, 2.1, and 2.3. Each of these values appears as a level on a frequency table. If you decide to round each value to a single digit, include the statement

   format x 1.;
in the PROC FREQ step. Now the table lists the frequency count for formatted level 1 as two and formatted level 2 as three.

PROC FREQ treats formatted character variables in the same way. The formatted values are used to group the observations into the levels of a frequency table or crosstabulation table. PROC FREQ uses the entire value of a character format to classify an observation.

You can also use the FORMAT statement to assign formats that were created with PROC FORMAT to the variables. User-written formats determine the number of levels for a variable and provide labels for a table. If you use the same data with different formats, then you can produce frequency counts and statistics for different classifications of the variable values.

When you use PROC FORMAT to create a user-written format that combines missing and nonmissing values into one category, PROC FREQ treats the entire category of formatted values as missing. For example, a questionnaire codes answers as follows: 1 as yes, 2 as no, and 8 as no answer. The following PROC FORMAT step creates a user-written format:

proc format;
   value questfmt 1='Yes'

When you use a FORMAT statement to assign QUESTFMT. to a variable, the variable's frequency table no longer includes a frequency count for the response of 8. You must use MISSING or MISSPRINT in the TABLES statement to list the frequency for no answer. The frequency count for this level will include observations with either a value of 8 or a missing value (.).

The frequency or crosstabulation table lists the values of both character and numeric variables in ascending order based on internal (unformatted) variable values unless you change the order with the ORDER= option. To list the values in ascending order by formatted values, use ORDER=FORMATTED in the PROC FREQ statement.

For more information on the FORMAT statement, see SAS Language Reference: Dictionary.

Computational Resources
For each variable in a table request, PROC FREQ stores all of the levels in memory. If all variables are numeric and not formatted, this requires about 84 bytes for each variable level. When there are character variables or formatted numeric variables, the memory that is required depends on the formatted variable lengths, with longer formatted lengths requiring more memory. The number of levels for each variable is limited only by the largest integer that your operating environment can store.

For any single crosstabulation table requested, PROC FREQ builds the entire table in memory, regardless of whether the table has zero cell counts. Thus, if the numeric variables A, B, and C each have 10 levels, PROC FREQ requires 2520 bytes to store the variable levels for the table request A*B*C, as follows:

3 variables*10 levels/variable*84 bytes/level
In addition , PROC FREQ requires 8000 bytes to store the table cell frequencies
1000 cells * 8 bytes/cell 
even though there may be only 10 observations.

When the variables have many levels or when there are many multiway tables, your computer may not have enough memory to construct the tables. If PROC FREQ runs out of memory while constructing tables, it stops collecting levels for the variable with the most levels and returns the memory that is used by that variable. The procedure then builds the tables that do not contain the disabled variables.

If there is not enough memory for your table request and if increasing the available memory is impractical, you can reduce the number of multiway tables or variable levels. If you are not using CMH or AGREE in the TABLES statement to compute statistics across strata, reduce the number of multiway tables by using PROC SORT to sort the data set by one or more of the variables or use the DATA step to create an index for the variables. Then remove the sorted or indexed variables from the TABLES statement and include a BY statement that uses these variables. You can also reduce memory requirements by using a FORMAT statement in the PROC FREQ step to reduce the number of levels. Additionally, reducing the formatted variable lengths reduces the amount of memory that is needed to store the variable levels. For more information on using formats, see Grouping with Formats .

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