It is usually the complement of the null hypothesis.
In the second step of the procedure we identify the kind of data that is expected if the null hypothesis is true.
is also the power of the test when the null hypothesis, H0, is true.
By convention, if there is less than 5% chance of getting the observed differences by chance, we reject the null hypothesis and say we found a statistically significant difference between the two groups.
From the result of Levene's Test for Equality of Variances, we can reject the null hypothesis that there is no difference in the variances between the groups and accept the alternative hypothesis that there is a statistically significant difference in the variances between groups. The effect of not being able to assume equal variances is evident in the final column of the above figure where we see a reduction in the value of the tstatistic and a large reduction in the degrees of freedom (df). This has the effect of increasing the pvalue above the critical significance level of 0.05. In this case, we therefore do not accept the alternative hypothesis and accept that there are no statistically significant differences between means. This would not have been our conclusion had we not tested for homogeneity of variances.
One can never prove the truth of a statistical (null) hypothesis.
This is to keep the investigator from changing the criterion after the data have been examined. If the study resultin this case a betweengroup difference in mean cholesterol levels falls in the most extreme 5% of the theoretical sampling distribution corresponding to the null hypothesis, then the null hypothesis is rejected.
We usually use a ttest for a study of this design. Using our example of a clinical trial of lovastatin, the pvalue would be interpreted as the chance of obtaining a betweengroup difference in mean cholesterol levels as large or larger than that which was observed solely through sampling error from a theoretical distribution of between group differences that had a true mean of zero (i.e. the null hypothesis).
Null hypothesis significance testing uses the laws
Hypothesis testing is very important in the scientific community and is necessary for advancing theories and ideas. Statistical hypothesis tests are not just designed to select the more likely of two hypotheses—a test will remain with the null hypothesis until there's enough evidence to support the alternative hypothesis. Now you have seen several examples of hypothesis testing and you can better understand why it is so important. For more information on types of hypotheses see .
We decide: "The data (and its sample mean) are significantly different than the value of the mean hypothesized under the null hypothesis, at the .01 level of significance." This decision is likely to be wrong (Type I error) 1 time out of 100.
Formulation of a hypothesis, uses of hypothesis in research use

Null Hypothesis (from Internet Glossary of Statistical …
The probability of the rejecting the null hypothesis increases with the difference between population means.

but one difference is that ADF test uses null hypothesis that a ..
Therefore, if the null hypothesis is true , the level of the test, is the probability of a type I error.

5 Differences between Null and Alternative Hypothesis …
The probability of a type II error depends on the way the null hypothesis is false.
Statistical hypothesis testing  Wikipedia
where the observed sample mean, μ_{0} = value specified in null hypothesis, s = standard deviation of the sample measurements and n = the number of differences.
What is a Null Hypothesis?  Definition & Examples  …
where the observed sample mean difference, μ_{0} = value specified in null hypothesis, s_{d} = standard deviation of the differences in the sample measurements and n = sample size. For instance, if we wanted to test for a difference in mean SAT Math and mean SAT Verbal scores, we would random sample subjects, record their SATM and SATV scores in two separate columns, then create a third column that contained the differences between these scores. Then the sample mean and sample standard deviation would be those that were calculated on this column of differences.
Hypothesis Testing  Kean University
Notice that the top part of the statistic is the difference between the sample mean and the null hypothesis. The bottom part of the calculation is the standard error of the mean.
Significance Tests / Hypothesis Testing  Jerry Dallal
We decide: "The data (and its sample mean) are significantly different than the value of the mean hypothesized under the null hypothesis, at the .001 level of significance." This decision is likely to be wrong (Type I error) 1 time out of 1000.
Significance Tests / Hypothesis Testing
The pvalue is p = 0.236. This is not below the .05 standard, so we do not reject the null hypothesis. Thus it is possible that the true value of the population mean is 72. The 95% confidence interval suggests the mean could be anywhere between 67.78 and 73.06.
Suppose someone suggests a hypothesis that a certain population is 0
You need descriptive statistics for three reasons. First, if you don’t have enough variance on the variables of interest, you can’t test your null hypothesis. If everyone is white or no one is obese, you don’t have the right dataset for your study. Second, you are going to need to include a table of sample statistics in your paper. This should include standard demographic variables – age, sex, education, income and race are the main ones. Last, and not necessarily least, descriptive statistics will give you some insight into how your data are coded and distributed.