Answer
$1.Street\:Interviews:\:Not\:all\:people\:have\:an\:equal\:chance\:of\:being\:selected.\\2.Call-In\:Polls:\:Only\:those\:who\:are\:strongly\:interested\:or\:heard\:the\:broadcast\:respond.\\3.Mail\:Surveys:\:Only\:interested\:or\:available\:individuals\:respond.$
Work Step by Step
$In\:random\:sampling,\:the\:basic\:requirement\:is\:that,\:for\:a\:sample$
$\:of\:size,\:all\:possible\:samples\:of\:this\:size\:have\:an\:equal\:chance\:of\:$
$being\:selected\:from\:the\:population.\:There\:are\:several\:incorrect\:$
$methods\:commonly\:used\:by\:various\:researchers\:and\:agencies\:to\:$
$gain\:information\:are\:as\:follows\\$
$1.\:One\:incorrect\:method\:commonly\:used\:it\:to\:ask\:the\:person\:on\:$
$the\:street.\:News\:reporters\:use\:this\:technique\:quite\:often.\:Selecting\:$
$people\:haphazardly\:on\:the\:street\:does\:not\:meet\:the\:requirement\:$
$for\:simple\:random\:sampling,\:since\:not\:all\:possible\:samples\:of\:$
$a\:specific\:size\:have\:an\:equal\:chance\:of\:being\:selected.\:Many\:$
$people\:will\:be\:at\:work\:when\:the\:interview\:is\:being\:conducted\:$
$and\:therefore\:do\:not\:have\:a\:chance\:of\:being\:selected.$
$\\2.\:Another\:incorrect\:technique\:is\:to\:ask\:a\:question\:by\:either\:radio\:or\:$
$television\:and\:have\:the\:listeners\:or\:viewers\:call\:the\:station\:to\:give\:their\:$
$responses\:or\:opinions.\:Again,\:this\:sample\:is\:not\:random,\:since\:only\:$
$those\:who\:feel\:strongly\:for\:or\:against\:the\:issue\:may\:respond\:and\:$
$people\:may\:not\:have\:heard\:or\:seen\:the\:program.$
$\\3.\:A\:third\:erroneous\:method\:is\:to\:ask\:people\:to\:respond\:by\:mail.\:$
$Again,\:only\:those\:who\:are\:concerned\:and\:who\:have\:the\:time\:are\:$
$likely\:to\:respond.$
