BELL-SHAPED CURVE

The bell-shaped curve is also known as the normal curve of distribution. The concept behind the curve is that a sufficient number of randomly selected observations will fall in the middle, producing the shape of a bell.

The bell-shaped curve lays the groundwork for performing a host of statistical calculations. These calculations include sampling, probability, frequency distribution, and dispersion.

When actually taking the observations, the plotting of the observations may be skewed or asymmetrical. This reflects bias in the selection.

In a normal curve, the values fall within a specific area under the curve. For a normal curve, 68 percent of the values fall between +1 sigma; 95 percent, between +2 sigma; and 99 percent, between +3 sigma.

for Developing and Using a Bell-Shaped Curve

• Obtain a frequency distribution.
• Determine the class intervals for grouping the data.
• Draw an x-axis (horizontal line) to reflect the intervals.
• Draw a y-axis (vertical line) to reflect the cumulate frequency of occurrence for each interval.
• Plot the frequency of occurrences.
• Determine the purpose of generating a bell-shaped curve.
• Determine the skewness.
• If the observations produce a skewed or asymmetrical curve, then use the median to determine the measure of central tendency.
• If the observations produce a bell-shaped or symmetrical curve, then use the mean to determine the measure of central tendency.