(D) (Standard deviation) (790 - Park 22nd)MEASURES OF CENTRAL TENDENCY:Measures of central tendency: Imply a value in the distribution, around which the other values are distributed. It gives a mental picture of the central value.Mean - Widely used and is the most useful statistical average.> To obtain mean, individual observations are added together & divided by the number of observations.> Advantage - Easy to calculate and understand.> Disadvantage - May be unduly influenced by abnormal or extreme values in the distribution.Median -> To obtain median, data is arranged in an ascending or descending order of magnitude and then the value of the middle observation is located, which is called the median.> If the number of items or values is even, the practice is to take the average of the two middle values.> Advantage - not unduly influenced by abnormal or extreme values in the distribution.> Used for numeric data that is skewed and for ordinal data.> Most preferred measured of central tendency.> Application: In cumulative frequency graph and survival analysisMode -> It is the most frequently occurring value in a series of value or observation.> Advantages: It is easy to understand, and is not (or least) affected by the extreme items.> It is the only average that can be applied to qualitative data.> Mode = Mean - 3 (Mean - Median)> There can be more than one mode for a series of data and even no mode in series of data.> In a frequency polygon, mode can be located from the point where be curve takes a turn from increase to decrease.> Used primarily for bimodal distribution> Application - Practical epidemiological work (E.g. Determining peak of disease occurrence in investigation of disease outbreak.Measures of variation (or 'disperson*) are:1. Range2. Mean or Average Deviation3. Standard Deviation