knitr::opts_chunk$set( message = FALSE, warning = FALSE, collapse = TRUE, comment = "#>", fig.path = "man/figures/README-")
zscorer
do?The zscorer
packages facilitates the calculation of a range of anthropometric z-scores (i.e. the number of standard deviations from the mean) and adds them to survey data:
Weight-for-length (wfl) z-scores for children with lengths between 45 and 110 cm
Weight-for-height (wfh) z-scores for children with heights between 65 and 120 cm
Length-for-age (lfa) z-scores for children aged less than 24 months
Height-for-age (hfa) z-scores for children aged between 24 and 228 months
Weight-for-age (wfa) z-scores for children aged between zero and 120 months
Body mass index-for-age (bfa) z-scores for children aged between zero and 228 months
MUAC-for-age (mfa) z-scores for children aged between 3 and 228 months
Triceps skinfold-for-age (tsa) z-scores for children aged between 3 and 60 months
Sub-scapular skinfold-for-age (ssa) z-scores for children aged between 3 and 60 months
Head circumference-for-age (hca) z-scores for children aged between zero and 60 months
The z-scores are calculated using the WHO Child Growth Standards1,2 for children aged between zero and 60 months or the WHO Growth References3 for school-aged children and adolescents. MUAC-for-age (mfa) z-scores for children aged between 60 and 228 months are calculated using the MUAC-for-age growth reference developed by Mramba et al. (2017)4 using data from the USA and Africa. This reference has been validated with African school-age children and adolescents. The zscorer
comes packaged with the WHO Growth References data and the MUAC-for-age reference data.
You can install zscorer
from CRAN:
install.packages("zscorer")
or you can install the development version of zscorer
from GitHub with:
if(!require(remotes)) install.packages("remotes") remotes::install_github("nutriverse/zscorer")
then load zscorer
# load package library(zscorer)
The main function in the zscorer
package is addWGSR
.
To demonstrate its usage, we will use the accompanying dataset in zscorer
called anthro3
. We inspect the dataset as follows:
head(anthro3)
which returns:
head(anthro3)
anthro3
contains anthropometric data from a Rapid Assessment Method (RAM) survey from Burundi.
Anthropometric indices (e.g. weight-for-height z-scores) have not been calculated and added to the data.
We will use the addWGSR
function to add weight-for-height (wfh) z-scores to the example data:
svy <- addWGSR(data = anthro3, sex = "sex", firstPart = "weight", secondPart = "height", index = "wfh")
A new column named wfhz has been added to the dataset:
head(svy)
The wfhz
column contains the weight-for-height (wfh) z-scores calculated from the sex
, weight
, and height
columns in the anthro3
dataset. The calculated z-scores are rounded to two decimals places unless the digits
option is used to specify a different precision (run ?addWGSR
to see description of various parameters that can be specified in the addWGSR
function).
The addWGSR
function takes up to nine parameters to calculate each index separately, depending on the index required. These are described in the Help files of the zscorer
package which can be accessed as follows:
?addWGSR
The standing parameter specifies how “stature” (i.e. length or height) was measured. If this is not specified, and in some special circumstances, height and age rules will be applied when calculating z-scores. These rules are described in the table below.
+---------------+---------------+---------------+---------------+----------------------------------------+ | index | standing | age | height | Action | +===============+===============+===============+===============+========================================+ | hfa or lfa | standing | < 731 days | | index = lfa | | | | | | height = height + 0.7 cm | +---------------+---------------+---------------+---------------+----------------------------------------+ | hfa or lfa | supine | < 731 days | | index = lfa | +---------------+---------------+---------------+---------------+----------------------------------------+ | hfa or lfa | unknown | < 731 days | | index = lfa | +---------------+---------------+---------------+---------------+----------------------------------------+ | hfa or lfa | standing | ≥ 731 days | | index = hfa | +---------------+---------------+---------------+---------------+----------------------------------------+ | hfa or lfa | supine | ≥ 731 days | | index = hfa | | | | | | height = height - 0.7 cm | +---------------+---------------+---------------+---------------+----------------------------------------+ | hfa or lfa | unknown | ≥ 731 days | | index = hfa | +---------------+---------------+---------------+---------------+----------------------------------------+ | wfh or wfl | standing | | < 65 cm | index = wfl | | | | | | height = height + 0.7 cm | +---------------+---------------+---------------+---------------+----------------------------------------+ | wfh or wfl | standing | | ≥ 65 cm | index = wfh | +---------------+---------------+---------------+---------------+----------------------------------------+ | wfh or wfl | supine | | ≤ 110 cm | index = wfl | +---------------+---------------+---------------+---------------+----------------------------------------+ | wfh or wfl | supine | | more than | index = wfh | | | | | 110 cm | height = height - 0.7 cm | +---------------+---------------+---------------+---------------+----------------------------------------+ | wfh or wfl | unknown | | < 87 cm | index = wfl | +---------------+---------------+---------------+---------------+----------------------------------------+ | wfh or wfl | unknown | | ≥ 87 cm | index = wfh | +---------------+---------------+---------------+---------------+----------------------------------------+ | bfa | standing | < 731 days | | height = height + 0.7 cm | +---------------+---------------+---------------+---------------+----------------------------------------+ | bfa | standing | ≥ 731 days | | height = height - 0.7 cm | +---------------+---------------+---------------+---------------+----------------------------------------+
The addWGSR()
function will not produce error messages unless there is something very wrong with the data or the specified parameters. If an error is encountered in a record then the value NA is returned. Error conditions are listed in the table below.
+--------------------------------------------------+----------------------------------------+
| Error condition | Action |
+==================================================+========================================+
| Missing or nonsense value in standing
parameter| Set standing
to 3
(unknown) and |
| | apply appropriate height or age rules. |
+--------------------------------------------------+----------------------------------------+
| Unknown index
specified | Return NA for z-score. |
+--------------------------------------------------+----------------------------------------+
| Missing sex
| Return NA for z-score. |
+--------------------------------------------------+----------------------------------------+
| Missing firstPart
| Return NA for z-score. |
+--------------------------------------------------+----------------------------------------+
| Missing secondPart
| Return NA for z-score. |
+--------------------------------------------------+----------------------------------------+
| sex
is not male (1
) or female (2
) | Return NA for z-score. |
+--------------------------------------------------+----------------------------------------+
| firstPart
is not numeric | Return NA for z-score. |
+--------------------------------------------------+----------------------------------------+
| secondPart
is not numeric | Return NA for z-score. |
+--------------------------------------------------+----------------------------------------+
| Missing thirdPart
when index = "bfa"
| Return NA for z-score. |
+--------------------------------------------------+----------------------------------------+
| thirdPart
is not numeric when index = "bfa"
| Return NA for z-score. |
+--------------------------------------------------+----------------------------------------+
| secondPart
is out of range for specified index | Return NA for z-score. |
+--------------------------------------------------+----------------------------------------+
We can see this error behaviour using the example data:
table(is.na(svy$wfhz))
We can display the problem record:
svy[is.na(svy$wfhz), ]
The problem is due to the value 9 in the sex
column, which should be coded 1 (for male) and 2 (for female). Z-scores are only calculated for records with sex specified as either 1 (male) or 2 (female). All other values, including NA, will return NA.
The addWGSR()
function requires that data are recorded using the required units or required codes (see ?addWGSR
to check units required by the different function parameters).
The addWGSR()
function will return incorrect values if the data are not recorded using the required units. For example, this attempt to add weight-for-age z-scores to the example data:
svy <- addWGSR(data = svy, sex = "sex", firstPart = "weight", secondPart = "age", index = "wfa")
will give incorrect results:
summary(svy$wfaz)
The odd range of values is due to age being recorded in months rather than days.
It is simple to convert all ages from months to days:
svy$age <- svy$age * (365.25 / 12) head(svy)
before calculating and adding weight-for-age z-scores:
svy <- addWGSR(data = svy, sex = "sex", firstPart = "weight", secondPart = "age", index = "wfa") head(svy) summary(svy$wfaz)
The muac column in the example dataset is recorded in millimetres (mm). We need to convert this to centimetres (cm):
svy$muac <- svy$muac / 10 head(svy)
before using the addWGS()
function to calculate MUAC-for-age z-scores:
svy <- addWGSR(svy, sex = "sex", firstPart = "muac", secondPart = "age", index = "mfa") head(svy)
As a last example we will use the addWGSR()
function to add body mass index-for-age (bfa) z-scores to the data to create a new variable called bmiAgeZ with a precision of 4 decimal places as:
svy <- addWGSR(data = svy, sex = "sex", firstPart = "weight", secondPart = "height", thirdPart = "age", index = "bfa", output = "bmiAgeZ", digits = 4) head(svy)
To maintain support for earlier versions of the package, the earlier functions used to calculate anthropometric z-scores for weight-for-age
, height-for-age
and weight-for-height
still work but will be removed (defunct) in the next version of zscorer
. For current and new users, it is recommended to use addWGSR()
and getWGSR()
functions instead.
For this example, we will use the getWGS()
function and apply it to dummy data
of a 52 month old male child with a weight of 14.6 kg and a height of
98.0 cm.
# weight-for-age z-score waz <- getWGS(sexObserved = 1, # 1 = Male / 2 = Female firstPart = 14.6, # Weight in kilograms up to 1 decimal place secondPart = 52, # Age in whole months index = "wfa") # Anthropometric index (weight-for-age) waz # height-for-age z-score haz <- getWGS(sexObserved = 1, firstPart = 98, # Height in centimetres secondPart = 52, index = "hfa") # Anthropometric index (height-for-age) haz # weight-for-height z-score whz <- getWGS(sexObserved = 1, firstPart = 14.6, secondPart = 98, index = "wfh") # Anthropometric index (weight-for-height) whz
Applying the getWGS()
function results in a calculated z-score
for one child.
For this example, we will use the getCohortWGS()
function and apply it to sample
data anthro1
that came with zscorer
.
# Make a call for the anthro1 dataset
anthro1
As you will see, this dataset has the 4 variables you will need to use with
getCohortWGS()
to calculate the z-score
for the corresponding anthropometric
index. These are age
, sex
, weight
and height
.
library(zscorer)
head(anthro1)
To calculate the three anthropometric indices for all the children in the sample, we execute the following commands in R:
# weight-for-age z-score waz <- getCohortWGS(data = anthro1, sexObserved = "sex", firstPart = "weight", secondPart = "age", index = "wfa") head(waz, 50) # height-for-age z-score haz <- getCohortWGS(data = anthro1, sexObserved = "sex", firstPart = "height", secondPart = "age", index = "hfa") head(haz, 50) # weight-for-height z-score whz <- getCohortWGS(data = anthro1, sexObserved = "sex", firstPart = "weight", secondPart = "height", index = "wfh") head(whz, 50)
Applying the getCohortWGS()
function results in a vector of calculated z-scores
for all children in the cohort or sample.
For this example, we will use the getAllWGS()
function and apply it to sample
data anthro1
that came with zscorer
.
# weight-for-age z-score zScores <- getAllWGS(data = anthro1, sex = "sex", weight = "weight", height = "height", age = "age", index = "all") head(zScores, 20)
Applying the getAllWGS()
function results in a data frame of calculated z-scores
for all children in the cohort or sample for all the anthropometric indices.
To use the included Shiny app, run the following command in R:
run_zscorer()
This will initiate the Shiny app using the installed web browser in your current device as shown below:
knitr::include_graphics("man/figures/zscorerApp.png")
If you find the zscorer
package useful please cite using the suggested citation provided by a call to the citation
function as follows:
citation("zscorer")
Feedback, bug reports and feature requests are welcome; file issues or seek support here. If you would like to contribute to the package, please see our contributing guidelines.
This project is released with a Contributor Code of Conduct. By participating in this project you agree to abide by its terms.
1 World Health Organization. (2006). WHO child growth standards : length/height-for-age, weight-for-age, weight-for-length, weight -for-height and body mass index-for-age : methods and development. World Health Organization. https://apps.who.int/iris/handle/10665/43413
2 World Health Organization. (2007). WHO child growth standards : head circumference-for-age, arm circumference-for-age, triceps skinfold-for-age and subscapular skinfold-for-age : methods and development. World Health Organization. https://apps.who.int/iris/handle/10665/43706
3 de Onis M. Development of a WHO growth reference for school-aged children and adolescents. Bull World Health Org. 2007;85: 660–667. doi:10.2471/BLT.07.043497
4 Mramba L, Ngari M, Mwangome M, Muchai L, Bauni E, Walker AS, et al. A growth reference for mid upper arm circumference for age among school age children and adolescents, and validation for mortality: growth curve construction and longitudinal cohort study. BMJ. 2017;: j3423–8. doi:10.1136/bmj.j3423
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