When the population variances are unequal, we estimate each population variance with its sample variance.Hence, the unequal-variances test statistic of 
is approximately Student t-distributed with n1 + n2 -2 degrees of freedom.
Definitions:
Mood Disorders
A category of psychiatric disorders characterized by disturbances in mood or emotional state, including depression and bipolar disorder.
Biological Factors
Elements related to the physiological and genetic conditions that influence the behavior or health of an organism.
Genetic
Relating to genes or heredity, and the way in which certain qualities or diseases are passed down from parents to their offspring.
Neurochemical
A chemical, such as a neurotransmitter or neuromodulator, that plays a role in the functioning of the nervous system.
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