AD is a syndrome of acquired deficits in multiple domains of cognition which may progress to interference with everyday life deteriorating to inability to carry out activities of daily living. the profusion of cognitive,behavioral and psychiatric changes occurring during the course of the disease,are of clinical and practical importance as they may distress both the patients and caregivers,often precipitating institutionalization clinical drug trials conventionally report their results in terms of the differences between active and placebo treatments on a measured outcome together with its 95% confidence interval and statistical significance (p value < .05).because neither the observed difference nor its statistical significance may indicate clinical significance,the minimum clically important difference (MCID) has been suggested as a more useful measure of effectiveness. the MCID has been defined as "the smllest difference that patints perceive as beneficial and which would mandate, in the absence of trouble management. cohen's effect size is a distribution-based method to identify a clinically important change. Using this method a medium effect size of .5 is conventionnal. norman, solan and wyrwich suggested that the value of .5 be used as a default for defining important patient and clinician perceived change. the FDA suggested as early as 1989 that a reversal of the natural history of cognitive decline over 6 months constituted an MCID and that 1.4 MMSE points corresponds to the expected mean 6 month rate of decline in AD