Inaugural Lecture: Prof Hannelie Nel

Bringing dietary data closer to the truth with statistical adjustment: The 2018 Provincial Dietary Intake Survey as an example

A single 24-hour recall is often used as the primary instrument for measuring dietary intake in large dietary studies. A common concern with a single 24-hour recall is the day-to-day variation in the diet of free-living populations. The magnitude of the mostly random within-person variance varies by nutrients and is largely dependent on cultural and ecological factors. These errors result in large standard deviations in populations. A result of exaggerated variation is that the percentage of respondents below or above specified cut points will be distorted. A method to distinguish within-person from between-person variation, account for extreme intakes and allow for adjustment for covariates was applied.
Results indicated that the overall unadjusted and adjusted means of the nutrient intakes were mostly similar. However, large differences between unadjusted and the adjusted values of the percentiles of the intake of some nutrients were observed. The method compressed the distribution of nutrient intake towards the mean. This compression was most apparent when examining the percentage of the population below or above AMDR/DRI limits, especially for nutrients with skew distributions. In these cases, the adjusted percentage below or above the cut points was substantially smaller, demonstrating that the prevalence of dietary insufficiency or excess could be overestimated if intakes are not adjusted. Importantly, the method allows for estimation of population exposure without conducting repeated 24-hour recalls on the total sample. This study shows that a small subsample (11%) may be sufficient to conduct the described adjustments.

Bring dieetdata nader aan die waarheid deur statistiese aanpassing: Die 2018 Provinsiale Dieetinname-opname as ʼn voorbeeld

ʼn Enkele 24 uur-herroeping word gereeld as die primêre instrument vir die meting van dieetinname in groot dieetstudies gebruik. ʼn Algemene probleem met ʼn enkele 24 uur-herroeping is egter die daaglikse variasie in die dieet van vrylewende bevolkings. Die omvang van die hoofsaaklik lukrake intrapersoonlike variansie verskil tussen voedingstowwe en is hoofsaaklik afhanklik van kulturele en ekologiese faktore. Hierdie foute lei tot groot standaardafwykings in populasies. ʼn Resultaat van oordrewe variasie is dat die persentasie deelnemers onder of bo gespesifiseerde afsnypunte verwronge sal wees. ʼn Metode om intrapersoonlike variasie van interpersoonlike variasie te onderskei, ekstreemwaardes in ag te neem en voorsiening vir die aanpassing vir kovariate te maak, is toegepas.
Die resultate het getoon dat die algehele onaangepaste en aangepaste gemiddeldes van die inname van voedingstowwe hoofsaaklik soortgelyk was. Groot verskille is egter tussen die onaangepaste en die aangepaste waardes van die persentiele van die inname van sommige voedingstowwe waargeneem. Die metode het die verspreiding van voedingstofinname na die gemiddelde saamgepers. Hierdie samepersing was die duidelikste in die ondersoek na die persentasie van die populasie onder of bo AMDR/DRI grense, veral vir voedingstowwe met skewe verdelings. In hierdie gevalle was die aangepaste persentasie onder of bo die snypunte aanmerklik kleiner, wat toon dat die voorkoms van dieetgebrek of -oormaat oorskat kan word as innames nie aangepas word nie. Die metode maak voorts voorsiening vir raming van bevolkingsblootstelling sonder om herhaalde 24 uur-herroepings op die totale steekproef uit te voer. Hierdie studie toon dat ʼn klein substeekproef (11%) voldoende kan wees om die betrokke aanpassings uit te voer.

---
Videography & Production: Justin Alberts - Corporate Communication & Marketing Division
Email: [email protected]
---