Child Growth Velocity

[Mifa Nurfadilah]

Hi,
I would like to ask question how to calculate growth velocity (cm/month)
if the data i have:
child length at birth
child length at 6 mo
child length at 12 mo

is there any formula or model for it?
how to do it in stata?
thank you

[Andrew Musau]

As you do not have monthly data but biannual data, you need to make some assumptions regarding the child's growth. If you assume that the child grows at a constant rate, then you can divide the difference between successive 6-month periods by 6.

* Example generated by -dataex-. For more info, type help dataex
clear
input float(child_id month height_cm)
1  6 65
1 12 76
1 18 81
1 24 88
2  6 60
2 12 72
2 18 78
2 24 84
end

xtset child_id month, delta(6)
gen wanted= (F.height- height)/6

Res.:

. l, sepby(child_id)

     +----------------------------------------+
     | child_id   month   height~m     wanted |
     |----------------------------------------|
  1. |        1       6         65   1.833333 |
  2. |        1      12         76   .8333333 |
  3. |        1      18         81   1.166667 |
  4. |        1      24         88          . |
     |----------------------------------------|
  5. |        2       6         60          2 |
  6. |        2      12         72          1 |
  7. |        2      18         78          1 |
  8. |        2      24         84          . |
     +----------------------------------------+

So between month 6 and month 12, the child with ID=1 grew at approximately 1.83 centimeters per month. I'm not a child growth specialist, but perhaps something other than an arithmetic average might be more appropriate.

[Andrew Musau]

This may be useful if your question was one of an empirical child growth model: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7511607/

[Mifa Nurfadilah]

Thank you Andrew for the code and the reference.
If i can have the monthly data, could you please explain how can i calculate the growth velocity in different way?

[Andrew Musau]

If you have monthly rates and want to calculate the effective rate over some \(n\)-period horizon, e.g., 6 months or 1 year, you can use the geometric mean formula.

$$\text{Effective rate} = \sqrt[n]{(1 + r_1) \times (1 + r_2) \times \dots \times (1 + r_n)} - 1$$

where \(r_1,r_2,\cdots, r_n\) are the individual rates and \(n\) is the number of periods (in your case, the number of monthly rates). But if you stick with raw changes in height as opposed to growth rates and follow #2, then it is simply a matter of changing the units in the code given.

xtset child_id month
gen wanted= F.height- height

and since a unit is a month, the change is expressed in units of cm/month.

[Mifa Nurfadilah]

Hi Andrew
thank you for the explanation,
It is really helppfull