Date posted: June 9, 2012

Dr Lizmy Ajith

Statistics is the field of science concerned with the collection, classification, summarising & interpretation of numerical data 

Biostatistics: application of statistical methods in biological sciences to deal with living things

Medical Statistics

Application of statistics in medicine

  • To compare efficacy of a drug
  • The % cured, relieved or died in experiment
  • To find an association B/W 2 attributes
  • In epidemiological studies 

Sources of data collection

  • Experiments
  • Surveys
  • Records 

Presentation of data

  • Tabulation
  • Diagrams 


  • The process of selecting a representative part from the whole
  • Sample: the representative part
  • Population: the whole from which sample is drawn 

Measures of central Tendency

  • Mean
  • Median
  • Mode 

Arithmetic average obtained by summing up all the observation & dividing the total by the No: of observations

X = ∑ X


Eg: E.S.R of 7 subjects – 7,5,3,4,6,4,5

Mean = 7+5+3+4+6+4+5    =  4.86

The middle observation when  all the observation are arranged in ascending or descending order

It implies the mid value of series.

Eg: E.S.R. of 7 subjects arranged in ascending order are, 3, 4, 4, 5, 5, 6,7

The 4th observation i.e. ‘5’ is the median

Most frequently occurring observation in a series

Eg: the size of induration in tuberculin test of 10 boys – 3, 5, 7, 7, 8, 8, 8, 10 ,11, 12

The mode here is ‘8’

Measures of variability
Variability: biological data collected by measurement or counting.

No 2 measurements in man are absolutely equal

Measures of variability of observations help to find how individual observations are dispersed around the mean

Measures of variability

  • Range
  • Mean deviation
  • Standard deviation
  • Standard error 

The normal limits of a biological characteristic

Eg: systolic B.P 100-140mm of Hg

Mean deviation
Mean of the absolute deviation from the central tendency

MD = ∑ X – X


Mean deviation about mean

                = sum of the absolute deviation from the mean / No: of observation

Standard deviation

Square root of the average of the sum of the squares of deviations taken from the mean

√ ∑ (x – x  )2


Indicates whether the variation of difference of an individual from the mean is by chance or real

Standard error

Difference between sample & population values

Is a measure of chance variation

SE = SD / √n

The relationship or association between two quantitatively measured variables.

Eg: relation between cholesterol level & B.P


  • Change in the measurements of a variable character
  • Helps to predict the value of one character from the knowledge of the other character
  • Eg: to estimate height when weight is known 

Probability or chance
It is a ratio of occurrence of favorable chances out of the total possible outcomes.

Eg: chances of one drug being better than the other

Test of significance

  • Mathematical methods by which the probability of an observed difference occurring by chance is found
  • The common test in use are ‘Z’ test, ‘t’ test & ‘Chi square’ test. 

Stages in performing a test of significance

  • State null hypothesis(Ho) : statement of no differenceEg: vitamins A & D makes no difference in growth
  • State alternate hypothesis(H1): vitamins A & D play a significant role in promoting growth
  • Determine the probabiility(P) of occurrence of your estimate
  • Draw conclusion on the basis of ‘P’ value i.e accept or reject the null hypothesis – decide whether the difference observed is due to chance or due to vitamins A & D  

Paired ‘t’ test

  • Applied to paired data of independent observations from one sample only when each individual gives a pair of observation
  • To study the effect of a drug before & after
  • To compare the effects of 2 drugs

t = x


Dr Lizmy ajith, B.H.M.S, M.D(HOM)
Dept. of Practice of Medicine,
Govt. Homoeopathic Medical College,Kozhikode


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