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Epi Info™ User Guide


Nutritional Anthropometry




Using the Nutrition Functions

There may be cases where you have already collected the nutritional data in another program (e.g., Microsoft Excel) and want to add the z-scores and percentiles to it. Re-entering all of this data into the Epi Info 7 Nutrition project in order to add the scores would be inefficient and time-consuming.

Epi Info 7’s Analysis module contains two specialized functions, ZSCORE and PFROMZ, that you can use to add z-scores and percentiles to an existing set of data very quickly. If the reader is not familiar with using analysis program editor code or functions, reading the Analysis chapter(s) is highly recommended.

The ZSCORE Function

The ZSCORE function takes a series of parameters and returns a z-score based on those parameters. These parameters include the:

  • Growth reference set to use (i.e., the CDC 2000 Growth Reference).
  • Type of z-score to calculate (i.e., body mass index-for-age).
  • Name of the column that stores the primary measurement (i.e., BMI).
  • Name of the column that stores the secondary measurement (i.e., AgeMonths).
  • Name of the column that stores the child’s gender, stored as 1s and 2s.

Each parameter is separated by a comma, and the entire series of parameters are enclosed in parenthesis. If the intention was to calculate body mass index-for-age using the CDC 2000 Growth Reference where the child’s raw BMI is stored in a column called BMI, the child’s age in months is stored in a column called AgeMonths. The child’s gender is stored in a column called Gender. The function would look like the following:

ZSCORE("CDC 2000", "BMI", BMI, AgeMonths, Gender)

The first parameter is “CDC 2000”. Notice the quotes that surround it. These are required for the first parameter to be accepted. It tells the program to calculate the z-score using the CDC 2000 Growth Reference. The full set of valid references that can be used for this parameter include those shown in Table 6.1.


Parameter value

Reference

CDC 2000

CDC 2000 Growth References

WHO CGS

WHO Child Growth Standards (0-5 years)

WHO 2007

WHO Reference 2007 (5-19 years)

NCHS 1977

CDC/WHO 1977 Growth Reference

Table 6.1: The valid values for the first parameter


The second parameter is “BMI”, which tells the program to calculate a z-score for body mass index-for-age. The quotes for this second parameter are also required. The full set of valid z-score types that can be used for the second parameter include those shown in table 6.2. Not all growth references support all types of z-score calculations (i.e., “BMI” may be valid for the second parameter, but it is not supported when using the NCHS 1977 growth reference).


Parameter value

Type of z-score

BMI

Body mass index-for-age

HtAge

Height-for-age

WtAge

Weight-for-age

WtHt

Weight-for-height

WtLgth

Weight-for-length

HeadCircum

Head circumference-for-age

LgthAge

Length-for-age

Ssf

Subscapular skinfold-for-age

Tsf

Triceps skinfold-for-age

Table 6.2: The valid values for the second parameter


The third parameter is also BMI, but does not contain any quotes because this parameter is the name of the column (or field) in the current data set that contains the child’s raw BMI calculation. For this example, it is assumed that the raw BMI score is stored in a column called BMI.


The fourth parameter is AgeMonths. It is critical to note that this parameter assumes the specified column is numeric and the numbers represent the child’s age in months.


The fifth parameter is Gender. It assumes the genders are stored in the database as 1s (male), and 2s (female).


Different databases may contain different column names than the example above. The final three parameters may vary considerably depending on what database is being read. However, the first two parameters will only ever accept the limited set of inputs displayed in Tables 6.1 and 6.2.

To run this calculation, the ZSCORE function must be paired with an ASSIGN command. For example:

ASSIGN BMIZ = ZSCORE("CDC 2000", "BMI", BMI, AgeMonths, Gender)

The above code tells Analysis to assign the z-score for body mass index-for-age using the CDC 2000 Growth Reference to the BMIZ column. It will do this for every row in the current dataset, effectively batch-processing all the rows. This should only take a few seconds even on very large data sets.

The PFROMZ Function

The second Nutritional Anthropometry function included with Epi Info 7 is called PFROMZ. You can use it after a z-score has been calculated and convert the z-score into a percentile. It is simple to use: Simply pass in the column name that contains a z-score as a parameter, and the corresponding percentile is returned. For example:

PFROMZ(BMIZ)

The above code assumes the BMIZ column contains the z-scores for body mass index-for-age. Note that the PFROMZ function does not need to know the reference, z-score type, gender, etc., of the data. To use it effectively, it must be combined with the ASSIGN command. For example:
ASSIGN BMIP = PFROMZ(BMIZ)

The above code tells Analysis to assign to the BMIP column the percentile associated with the values stored in the BMIZ column. It will do this for every row in the current data set, effectively batch-processing all of the rows and adding percentiles to each of them.
 

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  • Page last updated: June 6, 2012
  • Page created: June 6, 2012
  • Content source: Epi Info™
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