document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=1061848>Question 1061848</A>: <I><SPAN STYLE=\"word-break: break-all;\"> A certain virus infects one in every 500 people. A test used to detect the virus in a person is positive 90% of the time if the person has the virus and 10% of the time if the person does not have the virus. \r<BR>\n" );
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document.write( "What is the probability that a person has the virus given that they have tested positive.\r<BR>\n" );
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document.write( "What is the probability that a person does not have the virus given that they have tested negative.<BR>\n" );
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document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #677148 by </B><A HREF=/tutors/aboutme.mpl?userid=Boreal><B>Boreal(15235)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=Boreal><IMG SRC=http://www.algebra.com/images/aboutme-small.gif BORDER=0 alt=\"About Me\" VALIGN=MIDDLE></A>&nbsp;<IMG SRC=http://www.algebra.com/cgi-bin/show-face.mpl?user=Boreal><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=677148\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> Use 10000 people so 20 are ill.<BR>\n" );
document.write( "==========T+=======T-=======Total<BR>\n" );
document.write( "D+======18=======2=========20<BR>\n" );
document.write( "D--======998======8982=====9980<BR>\n" );
document.write( "Total======1016======8984==========10000<BR>\n" );
document.write( "The table shows that of the 20 who have the disease, 18 test positive and 2 negative.<BR>\n" );
document.write( "Of the 9980 who don't have the disease, 10% test positive and 90% negative.<BR>\n" );
document.write( "If the person tests negative, and there are 8984 of them, there is a 8982/8984 chance they don't have the virus (99.98%)<BR>\n" );
document.write( "If the person tests positive (1016), there is an 18/1016 chance they have the virus or 1.77%.<BR>\n" );
document.write( "In general, there is a 0.2% chance of a randomly selected individual having the disease, and if they test positive, that has increased nearly 9-fold to 1.77%, which is still low. Because so many people don't have the disease, the fact that 10% of them test positive means that there will be a lot of false positive tests. </SPAN>\n" );
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