Two standard deviations take in 95% of the pigs. Therefore, 1,206 pigs weigh between 6.5 lb. and 17.3 lb.

Three standard deviations will include 99% of the pigs, so 1,261 pigs weigh between 3.8 lb. and 20.0 lb. Of course, these numbers are approximations because data might not be perfectly, normally distributed, as Figure 1 shows, or because insufficient numbers of animals were weighed to adequately estimate these parameters.

• Coefficient of variation (CV): The coefficient of variation is calculated by dividing the standard deviation by the mean, then multiplying by 100.

Referring again to the example in Table 1, the standard deviation of 2.7 lb. can also be presented as a coefficient of variation of 22.7%. The standard deviation becomes larger as the pigs grow.

Therefore, in order to determine if relative variation is increasing or decreasing as pigs grow, the coefficient of variation is often used.

In Table 1, while the standard deviation increases as the pigs become heavier, the coefficient of variation decreases, indicating that relative variability among pigs under commercial conditions typically declines as they grow.

There is one danger in using CV to represent variation. Sometimes, within pigs of the same age, the CV changes not because the SD changes, but because the mean weight changes (i.e., weaning weight increases). One could misinterpret this as meaning variation is reduced, when in actuality the variation did not change at all.

Measuring Variation On the Farm

There is surprisingly little information on normal distributions of bodyweights on commercial farms. Because it requires the weighing of many animals in order to accurately estimate SD and CV, it is not commonly done on most farms.

Furthermore, most research data cannot be used as reference points because in most experiments, animals are pre-selected to obtain a uniform group to increase experimental precision.

Geneticists have this information because variation is essential in selection programs, but even that data is not widely available.

Still, it is important that we develop an understanding of “normal” or “typical” variation, because it helps us to develop strategies for dealing with problems on individual farms or systems.

Standards can be used to determine if a given farm is better advised to manage variation or to minimize it. The difference between these two choices represents a critical management decision. The limited amount of data on the subject suggests that much larger variation is seen in some circumstances. CVs of 20% to 35% in grow-finish pigs among commercial farms have been reported.

The minimum number of pigs that must be weighed in order to estimate a mean, a standard deviation or a coefficient of variation is not constant. Rather, the number depends on the intrinsic variability within the population.

At weaning, because variability is so high, weighing even 100 pigs provides a poor estimate of the CV. However, when pigs are removed from the nursery or at first pull in a finishing facility, randomly weighing as few as 50 pigs throughout the barn will provide satisfactory information.

Random selection for weighing is very important because weighing too many or too few of the lighter and heavier pigs within the group will skew the results. If too many “outliers” are weighed, variation will be overestimated; if too few are weighed, variation will be underestimated.

Using the weights of pigs at marketing does not provide an accurate estimate of variability because it represents a group of animals pre-selected according to their market weights. For this reason, we prefer to use the weights of pigs at first pull because all animals can be included. However, this is not a particularly convenient number to generate on most farms, so alternatives must be developed.

Causes of Variation

Many factors can affect the degree of variability observed on a given farm. Without question, a certain amount of variability is “programmed” at the time a pig is born. Pigs that are smaller at birth are compromised physiologically and socially, and their expected performance will fall short when compared to their heavier contemporaries.