Biostatistics- Measure of Dispersion, Standard deviation,Coefficient of variation
MEASURES OF DISPERSION:
- Dispersion is the degree of spread or variation of the variable about a central value.
USES:
- To determine the reliability of an average.
- To serve as basis for control of variability.
- To compare two or more series in relation to their variability.
- Facilitate further statistical analysis.
RANGE:
- It is the simplest method, Defined as the difference between the value of the smallest item and the value of the largest item.
- This measure gives no information about the values that lie between the extremes values.
- Subject to fluctuations from sample to sample.
MEAN DEVIATION:
- It is the average of the deviations from the arithmetic mean.
M.D = (ΣX – Xi)/n ,
- where Σ( sigma ) is the sum of, X is the arithmetic mean, Xi is the value of each observation in the data, n is the number of observation in the data.
STANDARD DEVIATION(SD):
- Most important and widely used.
- Also known as root mean square deviation, because it is the square root of the mean of the squared deviations from the arithmetic mean.
- Greater the standard deviation, greater will be the magnitude of dispersion from the mean.
- A small SD means a higher degree of uniformity of the observations.
CALCULATION:
- For ungrouped data:
- Calculate the mean(X) of the series.
- Take the deviations (d) of the items from the mean by : d=Xi – X, where Xi is the value of each observation.
- Square the deviations (d2) and obtain the total (∑ d2)
- Divide the ∑ d2 by the total number of observations i.e., (n-1) and obtain the square root. This gives the standard deviation.
- Symbolically, standard deviation is given by:
SD= √ ∑ d2 /(n-1)
- For grouped data with single units for class intervals:
S = √∑(Xi – X) x f i / (N -1)
Xi is the individual observation in the class interval
fi is the corresponding frequency
X is the mean
N is the total of all frequencies
- For grouped data with a range for the class interval:
S =√ ∑(Xi – X) x f i / (N -1)
Xi is the midpoint of the class interval
Confidence intervals:
- Lower limit & upper limit estimates for statistic given by:
- Lower Limit: statistic – C x SE (statistic)
- Upper Limit: statistic + C x SE.
-
Confidence coefficient:
- C = Confidence coefficient = 1.65 for 90% confidence interval
- C= 1.96 for 95% confidence interval
- C= 2.58 for 99% confidence interval
- C = 3.29 for 99.9% confidence interval
COEFFICIENT OF VARIATION(C.V.):
- A relative measure of dispersion.
- To compare two or more series of data with either different units of measurement or marked difference in mean.
C.V.= (Sx100)/ X
Where, C.V. is the coefficient of variation
S is the standard deviation
X is the mean
- Higher the C.V. greater is the variation in the series of data
Exam Important
Mean deviation is Measure of dispersion
Square root of deviation is also called as Standard deviation
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