A hypothesis can be classified into six types: simple, complex, associative and causal, directional, non-directional and null. In research, a hypothesis is characterized by three essential elements: variables, population and the correlation between the variables.
A simple hypothesis involves only two variables: one independent and one dependent. A complex hypothesis, meanwhile, involves multiple variables, such as two or more dependent and independent variables. A directional hypothesis is based on an established theory, while a non-directional hypothesis is used when there is insufficient scientific basis for the prediction. This type of hypothesis may be biased, as it does not necessarily state the nature of the relationship between the variables.
Researchers use a null hypothesis when they predict that the variables are not correlated. An associative hypothesis supposes that when one variable changes, the other variable also changes, while a causal hypothesis assumes the cause and effect relationship between variables.