Practice Consideration
There are several ways of describing an individual's food and/or nutrient consumption. The dietitian may use:
- A range
The area of variation between the lower and upper limits of intake. - An average
Results from the process of adding a series of item values and dividing the sum by the total numbers of items. - A weighted average
Is the average resulting from the multiplication of each value in a data set by a figure reflecting its significance.
Weighted average is necessary when dealing with frequency or distribution and its necessary to account for the fact their are multiple instances of some but not all of the values (as is the case in a nutrition analysis of a weekly dietary intake).
1. Range
Ranges provide information about the variation between the lower and upper limits of data.
EXAMPLE
Using a dietary intake in which no vegetables are eaten from Monday to Saturday and two and a half cups of cooked vegetables are eaten on Sunday, the range might be expressed in the following way:
"The vegetable intake ranges from 0 to 2 1/2 cups of vegetable per day compared to recommended intake of 2 1/2 cups per day."
That is,
Monday through Saturday = 0 Vegetable Exchanges* (0 cups cooked vegetables)
Sunday = 5 Vegetable Exchanges (2.5 cups cooked vegetables)
Consider how would you might express this information to other health professionals? Would you indicate that the intake for vegetables is adequate on Sunday, but less than optimal for the other 6 days of the week?
Describing a food intake pattern with extreme variation between the upper and lower limits of data is not useful in instances where average daily intake is required. The intake range is limited because it:
- Is sensitive to outliers in the data
- Considers only the smallest and largest data in the set and does not account for real patterns of consumption
- Shows a range for food exchange intake which would require the practitioner to provide ranges for the macronutrients and micronutrient intake too. This information is unhelpful when information on consistent intake is required
- Doesn't provide a single number for comparison with standards, e.g the My Plate recommendations
2. Average (of different intakes)
This information is most useful when the daily intakes across the week are weighted similarly.
EXAMPLE
Using the dietary intake in the example where no vegetables are eaten from Monday to Saturday and two and a half cups of cooked vegetables are eaten on Sunday, the average of different intakes is calculated in the following way.
Monday through Saturday = 0 Vegetable Exchanges* (0 cups cooked vegetables)
Sunday = 5 Vegetable Exchanges (2.5 cups cooked vegetables)
0 vegetable exchanges = 0
PLUS
5 vegetables exchanges = 5
Divided by 2 (for the number of different vegetable intakes) = 1 1/4 cups vegetable per day
Describing this intake pattern with an average is inaccurate because, again, the calculation is sensitive to extreme values and very large or very small numbers distort the data. This is particularly true when small sets of data are described. The average of different intakes is most useful when similar amounts of a food group are consumed from day to day. Based on this math you might conclude by saying:
"The vegetable intake is an average of 1 1/4 cups per day compared to recommended intake of 2 1/2 cups per day."
An average of different intakes has the advantage of providing a single number for comparison against a standard like My Plate, however it doesn't describe "usual intake" with accuracy.
3. A weighted average takes into consideration the number of days that a particular food intake pattern is consumed. This measure is useful in describing "usual intake" if the eating patterns vary widely during the week as is the case in our example.
EXAMPLE
Monday through Saturday = 0 Vegetable Exchanges* (0 cups cooked vegetables)
Sunday = 5 Vegetable Exchanges (2.5 cups cooked vegetables)
In this case the value for Monday through Saturday is 6 times as important as the value on Sunday since it occurs on 6 days versus one day.
1. Calculate the total number of servings per week
b. Add the totals from each dietary pattern together to obtain total per 7 days in the week
2. Calculate the average daily intake by dividing by 7 days per week
THIS EXAMPLE
a. Monday through Saturday: 0 vegetable exchanges x 6 days = 0
+ Sunday: 5 vegetables exchanges x 1 day = 5
Weekly total = 5 vegetable exchanges
b. Daily average = 5 divided by 7 days = 0.7 servings per day.
In this case dietary intake of vegetables may be described in the following way:
"The vegetable intake is an average of 2/3 cup per day compared to recommended intake of 2 1/2 cups per day."
Even though it is a bit more work to calculate the weighted average, this last measure provides useful information for comparing intake against standards.
* 1/2 cup cooked vegetables = 1 vegetable exchange