Answer
$Random,\:systematic,\:stratified\:and\:cluster$
Work Step by Step
$To\:obtain\:an\:unbiased\:sample,\:the\:statisticians\:have\:$
$developed\:several\:basic\:sampling\:methods.$
$\\The\:most\:common\:methods\:of\:sampling\:techniques\:are\:$
$random,\:systematic,\:stratified\:and\:cluster\:sampling.$