Prior to the widespread adoption of all-in, all-out (AIAO) production systems, variation in growth was largely a “hidden” cost. Pigs were sorted from pens when they reached market weight, and the fact that some took longer than others went largely unnoticed, or at least ignored.

Furthermore, in continuous-flow systems, downtime due to variable growth rates affects pen usage, while in AIAO systems it affects room or barn usage. Consequently, the economic impact of variation is much greater in AIAO systems.

Variation in market weights increases sort losses, a cost that goes straight to the bottom line. From a labor and management perspective, there is the annoyance factor of dealing with tail-end pigs in a room or barn, which is also greater than in pens.

The cost of variation has always been with us, but those costs are now much more obvious.

In the past, the industry's singular focus was on “average” growth rate. Today, while we continue to pay attention to growth rate averages, greater attention is being focused on the “range” in that growth ? the variability of growth.

Measuring Variation

Statistically, variation can be defined in a variety of ways. The most common terms used to express it are standard deviation (SD) and coefficient of variation (CV), although range may also be useful.

Most people are familiar with the “bell curve,” which shows the typical distribution of measurements within a specific group. Many measurements can be described using a bell curve (weight, height, etc.).

If the measurement of a group or population is made and, when plotted, displays a bell shape, it is called a “normal” distribution. If the plotted data do not follow the bell shape, the data are called “skewed.”

For example, when the body weights of 632 pigs, averaging 20 weeks of age, are charted (Figure 1), it shows a very typical distribution of body weights for pigs of that age. This distribution is almost “normal,” but is skewed slightly to the left, reflecting the exaggerated number of tail-enders in the group.

The shape of the bell curve reveals a great deal about a population or group. For example, if the bell shape is narrow, the population is relatively uniform because most of the measurements are closer to the average. If the bell shape is wide, the population is less uniform because more measurements are found further from the average.

One useful measure of the width of the bell curve is called the standard deviation (SD). The wider the bell shape, the larger the standard deviation, and the greater the variability of the group of animals.

Understanding the Terminology

Following are definitions of important statistics used to describe variation in a group of pigs.

• Mean: The mean is the average of all weights within a group of pigs. It provides no indication of the variability of weights within the group.

• Median: The median is determined by aligning all pig weights in order of magnitude (i.e., from smallest to largest or vice versa), then selecting the middle observation. If the distribution is “normal,” the median and the mean (average) will be very similar, if not identical.

• Minimum, maximum and range: The minimum and maximum are self-evident; they are the lightest and heaviest weights within the group. The difference between the minimum and maximum is called the range. The wider the shape of the bell curve, thus, the less uniform the group of pigs, the larger will be the range.

• Standard deviation (SD): The standard deviation is a measure of dispersion. The greater the variation in weight of a group of pigs, the larger will be the standard deviation.

In a “normal” distribution, statisticians have determined that one (1) standard deviation about the mean will include 68% of the pigs in the total group. Using the data in Table 1 for the 19-day-old pigs, and assuming the data is distributed normally, the standard deviation has been calculated to be 2.7 lb., with a mean of 11.9 lb. Thus, we can estimate that 68% of the pigs, or 863 pigs in this group, will be within one standard deviation of the mean and weigh between 9.2 lb. (11.9 - 2.7) and 14.6 lb. (11.9 + 2.7).