P-value  Is defined as: the probability of getting a difference at least as big as that observed if the null hypothesis is true.

P- value  The smaller the P-value, the stronger the evidence against the null hypothesis P-values ≤ 0.05 statistically significant P-values > 0.05 not significant

P-value If P<0.05 then the 95% confidence interval Will not contain the null value.

One sided vs two sided p-value Two sidedOne sided The investigator dose not know whether the parameter will be > or < the hypothetical value The investigator is sure that the mean may be either > or< the hypothetical value Eg. Study of the effect of a new additive on bioavailability Eg. Adding surfactant & measure solubility of substance

The two sided P-value Is twice the one-sided P-value One sided vs two sided p-value

Calculation of P-value by use t-test  Unpaired t test  Paired t test t= X 1 – X 2 /S √ ( 1/n 1 + 1/n 2 ) t = X1 –X2 / S√ 1/N

An approximate P-value corresponding to different values of the t-test derived from P- table Calculation of P-value by use t-test..cont

Example  The table shows the birth weights of children born to 14 heavy smokers (gp 1 ) and to 15 non smokers (gp 2), sampled from live births at large teaching hospital.  Is there a significant different between the 2 gp? Gp 2Gp

gp 1 X 1 = S 1 = N 1 =14 g gp 2 X 2 =3.626 S 2 =0.358 N 2 =15 t= ( )/0.4121√(1/14+1/15) t =-2.95 Df=27 P= ≤0.05

Exercise  The duration of loss of righting reflex in minutes was measured in 9 mice following treatment with barbiturates administration in the morning & afternoon on 2 different occasions.  Is there a significant different ? PM X2AM X1Mouse no