People sometimes ask why the app doesn't flag children as being malnourished or overweight based on their measurements. The short answer is that there are many competing standards for what is considered over- or under-weight and without considering child's overall health and family history the app would be wrong and stressful more often than it is right or useful.
The CDC Recommendations for interpreting percentiles are the following steps:
In contrast, the WHO defines cutoffs in terms of Z score (here). These cutoffs are:
The use of -2 Z-scores as a cut-off implies that 2.3% of the reference population will be classified as malnourished even if they are truly “healthy” individuals with no growth impairment.This means that about 5% of the normal, healthy children in the WHO growth reference would be "flagged" by these cutoffs as being malnourished or overweight. That doesn't mean there is necessarily anything wrong with them, it's simply a guideline for when their doctor should check a bit more closely whether they're growing well. This is why it's premature for an app to provide this type of concrete "your child is malnourished!" diagnosis to parents.
Other countries have different standards for which curves to use and what cutoffs to use with them that make applying a universal standard difficult. For example, what WHO cutoff should a Swedish family living in Brazil use? The Swedish one, Brazilian one, or the international WHO one? It is much better to simply provide the accurate percentile values from the selected curve and leave the diagnosis to a doctor.
These guidelines are meant for healthcare professionals, not apps and parents. When a pediatrician sees a measurement that is flagged by a cutoff rule, she may (or may not) order some tests or other diagnostics to see if there really is an issue. When a parent sees a measurement flagged by a cutoff rule, what action would they take? At best, they would calmly go see the doctor (who was already monitoring the child anyway). In reality most parents would worry unnecessarily there is something wrong with their child. At worst they may try to force or withhold food or take other ill-advised actions to get their child to be the "right" size.
Parents worry about their children's health, this is natural. Arbitrary cutoffs, scores, and categories can make this get out of hand (see the "what is a percentile" page, it's not a contest!). I have had parents write in concerned that since the app only displays the Z score to the nearest 0.1 their child whose "healthy" Z score was -0.98 would be displayed as a "bad" Z score of -1.0 and be classified as malnourished. This is ridiculous, the difference between Z = -0.98 and Z = -1.0 is a few grams - the difference between a baby with a full stomach or a baby who recently filled their diaper.
Which is more concerning? A child born with a Z value of 1.0 who rapidly declines to a Z value of -1.5 over 3 months or a child born with a Z value of -1.8 who fluctuates around that Z value and happens to be at Z = -2 for one measurement? Answer: You can't tell just from the Z score! They both could warrant follow up from their doctor but the first case is arguably more likely to be a real problem - and is also the case that wouldn't have been flagged with an arbitrary cutoff.