Why do we find probability of deviation in biology ? And when can the null hypothesis be accepted

probability is used to predict the chance that an event is likely to occur at a particular chosen level of significance.

Null hypothesis is accepted in biology when the p value is less than 1%, statistically, when the value of p<0.01

Explanation:

Determing the significance level is decided by the researcher, often times, scientists commonly use the 0.05, 0.01, or 0.001 probability levels as cut-off values. For instance, in the example experiment, you used the 1% probability. Thus, P ≥ 0.01 can be interpreted to mean that chance likely caused the deviation between the observed and the expected values (i.e. there is a greater than 1% probability that chance explains the data).

If instead we had observed that P ≤ 0.01, this would mean that there is less than a 1% probability that our data can be explained by chance. There is a significant difference between our expected and observed results, so the deviation must be caused by something other than chance.