Thank you for the response. Are there any "expiration date" to prereq courses? As in, is it less competitive to have prereqs that are more than certain number of years old? I know that for some professional schools strongly prefer (or even require) prereq classes that are within 5 years of marticulation, but I haven't been able to find any information on whether or not PhD programs have a such preference.
Also, do biostat PhD programs have same prerequisites as statistics PhD?