Biostatistics- Measure of Dispersion, Standard deviation,Coefficient of variation

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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