Convert to/from measurement from/to z-score with growth reference
A function to convert between measurements and z-scores using a growth reference previously fitted by the LMS method.
LMS2z(x, y, sex, measure, ref, toz = TRUE, LMStable = FALSE)
x: vector of ages in units of years.y: vector or one-column matrix of either measurements or z-scores, depending on the value of toz.sex: vector where 1/2 = males/females = boys/girls = TRUE/FALSE, based on the uppercased first character of the string.measure: unique measurement name, as character string, the choice depending on the choice of ref (see e.g. references uk90, who06 and ukwhopt).ref: unique growth reference, either as name or character string, available as a data object or data frame (e.g. uk90, who06 or ukwhopt).toz: logical set to TRUE for conversion from measurement to z-score, or FALSE for the reverse.LMStable: logical set to TRUE to return the associated LMS table as a data frame in attribute LMStable.A vector or matrix containing the transformed values. If y is a vector then a vector of length(x) is returned, else if y is a one-column matrix then a matrix is returned, with length(x) rows and length(y) columns. The matrix row names are set to x, and the column names to either y or if toz is FALSE, z2cent(y). If LMStable is TRUE the associated LMS table is returned as a data frame in attribute LMStable.
Growth references fitted by the LMS method consist of a table of L, M and S values by age and sex. Vectors of L, M and S corresponding to x and sex are extracted using cubic interpolation and passed to either cLMS or zLMS, depending on toz.
Disjunct references are supported, where there is a disjunction in the centiles at a particular age. This may be because the measurement changes, e.g. from length to height, or because two different references have been joined together. The disjunction is flagged by including two rows at the common age, but with different L, M and S values, and measurements at this age are ascribed to the older reference. For example the who06
reference has a disjunction at 2 years reflecting the switch from length to height. As a result height at just below and just above 2 years returns a different z-score.
## convert girls' heights data to UK 90 z-scores data(heights) data(uk90) with(heights, LMS2z(age, height, sex = 2, measure = 'ht', ref = 'uk90')) ## construct table of boys' weight centiles by age for WHO standard data(who06) zs <- -4:4*2/3 # z-scores for 9 centiles ages <- 0:20/4 # 3-month ages to 5 years LMS2z(ages, as.matrix(zs), sex = 'm', measure = 'wt', ref = who06, toz = FALSE, LMStable = TRUE)
z2cent. The LMS method can be fitted to data using the package gamlss with the BCCG or BCCGo family, where nu (originally lambda), mu and sigma correspond to L, M and S respectively.
Tim Cole tim.cole@ucl.ac.uk