I don't understand three important issues :

(1) Why do we calculate the test statistic under the null hypothesis ?

(2) Why we reject the null hypothesis when :

p-value < significance level ?

(3) Why type-I error is more serious than type-II error ?

I don't understand three important issues :

(1) Why do we calculate the test statistic under the null hypothesis ?

(2) Why do we reject the null hypothesis when :

p-value < significance level ?

(3) Why is type-I error more serious than type-II error ?

I don't understand three important issue :

(1) Why we calculate test statistic under null hypothesis ?

(2) Why we reject null hypothesis when :

p-value < significance level ?

(3) Why type-I error is more serious than type-II error ?

I don't understand three important issues :

(1) Why do we calculate the test statistic under the null hypothesis ?

(2) Why we reject the null hypothesis when :

p-value < significance level ?

(3) Why type-I error is more serious than type-II error ?