{"id":338,"date":"2015-07-12T08:36:00","date_gmt":"2015-07-12T07:36:00","guid":{"rendered":"http:\/\/pcool.dyndns.org:8080\/statsbook\/?page_id=338"},"modified":"2025-06-27T20:53:27","modified_gmt":"2025-06-27T19:53:27","slug":"epidemiology-2","status":"publish","type":"page","link":"https:\/\/pcool.dyndns.org\/index.php\/epidemiology-2\/","title":{"rendered":"Epidemiology"},"content":{"rendered":"\n<p><strong>Screening is the application of a test to asymptomatic individuals to assess the likelihood that an individual has a particular disease.<\/strong><\/p>\n\n\n\n<p><strong>Surveillance of disease is used to look for changes in trends or distribution of the disease.<\/strong><\/p>\n\n\n\n<div class=\"wp-block-mathml-mathmlblock\">\\(Prevalence = \\frac{Number Of Existing Cases Per Year}{PopulationAtRisk} \\)<script src=\"https:\/\/pcool.dyndns.org\/wp-includes\/js\/dist\/hooks.min.js?ver=dd5603f07f9220ed27f1\" id=\"wp-hooks-js\"><\/script>\n<script src=\"https:\/\/pcool.dyndns.org\/wp-includes\/js\/dist\/i18n.min.js?ver=c26c3dc7bed366793375\" id=\"wp-i18n-js\"><\/script>\n<script id=\"wp-i18n-js-after\">\nwp.i18n.setLocaleData( { 'text direction\\u0004ltr': [ 'ltr' ] } );\n\/\/# sourceURL=wp-i18n-js-after\n<\/script>\n<script  async src=\"https:\/\/cdnjs.cloudflare.com\/ajax\/libs\/mathjax\/2.7.7\/MathJax.js?config=TeX-MML-AM_CHTML\" id=\"mathjax-js\"><\/script>\n<\/div>\n\n\n\n<div class=\"wp-block-mathml-mathmlblock\">\\(Incidence = \\frac{NumberOfNewCasesPerYear}{PopulationAtRisk} \\)<\/div>\n\n\n\n<p><strong>Prevalence ~ Incidence x Mean disease duration<\/strong><\/p>\n\n\n\n<p><em>(when population and disorder stable)<\/em><\/p>\n\n\n\n<p><strong>A risk factor is an exposure or attribute that increases the probability of a disease.<\/strong><\/p>\n\n\n\n<table id=\"tablepress-1\" class=\"tablepress tablepress-id-1\">\n<thead>\n<tr class=\"row-1\">\n\t<td class=\"column-1\"><\/td><th class=\"column-2\">Disease<\/th><th class=\"column-3\">No Disease<\/th>\n<\/tr>\n<\/thead>\n<tbody class=\"row-striping row-hover\">\n<tr class=\"row-2\">\n\t<td class=\"column-1\">Exposure<\/td><td class=\"column-2\">a<\/td><td class=\"column-3\">b<\/td>\n<\/tr>\n<tr class=\"row-3\">\n\t<td class=\"column-1\">No Exposure<\/td><td class=\"column-2\">c<\/td><td class=\"column-3\">d<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<!-- #tablepress-1 from cache -->\n\n\n<p><strong>The absolute risk is always between zero and 1 and the relative risk is always larger than zero.<\/strong><\/p>\n\n\n\n<p><strong>Absolute Risk Exposed Individuals:<br><\/strong><\/p>\n\n\n\n<div class=\"wp-block-mathml-mathmlblock\">\\(ARexposure = \\frac{a}{a+b} \\)\n<\/div>\n\n\n\n<p><strong>Absolute Risk Non Exposed Individuals:<\/strong><\/p>\n\n\n\n<div class=\"wp-block-mathml-mathmlblock\">\\(ARnoexposure  = \\frac{c}{c+d} \\)<\/div>\n\n\n\n<p><strong>Relative Risk:<\/strong><\/p>\n\n\n\n<p><strong>The relative risk is the risk of developing the disease that is due to an exposure in the exposed group<\/strong><\/p>\n\n\n\n<div class=\"wp-block-mathml-mathmlblock\">\\(RR = \\frac{a \\over (a+b)}{c \\over (c+d)} \\)<\/div>\n\n\n\n<p><strong>Odds Ratio:<\/strong><\/p>\n\n\n\n<p><strong>The odds ratio is the ratio of the odds of a disease occurring among exposed individuals to that of it occurring in unexposed individuals.<\/strong><\/p>\n\n\n\n<div class=\"wp-block-mathml-mathmlblock\">\\(OR = \\frac{a \\cdot d}{b \\cdot c} \\)<\/div>\n\n\n\n<p><strong>An odds ratio or relative risk greater than 1 indicates the exposure is a risk factor for developing the disease. If the odds ratio or relative risk is 1, the exposure has no effect on the disease; if they are less than 1, the exposure protects against disease. <\/strong><\/p>\n\n\n\n<p>In the R console<sup class='sup-ref-note' id='note-zotero-ref-p338-r1-o1'><a class='sup-ref-note' href='#zotero-ref-p338-r1'>1<\/a><\/sup>:<\/p>\n\n\n\n<pre class=\"wp-block-code has-small-font-size\"><code><span style=\"color: #ff0000;\"><em>library(epiR)<\/em><\/span>\n<span style=\"color: #ff0000;\"><em>mat&lt;-matrix(c(a,c,b,d),ncol=2) <\/em> <span style=\"color: #000000;\">{enter values!}<\/span><\/span>\n<span style=\"color: #ff0000;\"><em>mat<\/em><\/span>\n<span style=\"color: #0000ff;\"><em>&#091;,1] &#091;,2]<\/em><\/span>\n<span style=\"color: #0000ff;\"><em> &#091;1,]&nbsp;&nbsp; a&nbsp; b<\/em><\/span>\n<span style=\"color: #0000ff;\"><em> &#091;2,]&nbsp;&nbsp; c&nbsp; d<\/em><\/span>\n<span style=\"color: #ff0000;\"><em> epi.2by2(mat)<\/em><\/span>\n<span style=\"color: #0000ff;\"><em> Outcome +&nbsp;&nbsp;&nbsp; Outcome -&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Total&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Inc risk *&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Odds<\/em><\/span>\n<span style=\"color: #0000ff;\"><em> Exposed +&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <\/em><\/span>\n<span style=\"color: #0000ff;\"><em> Exposed -&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <\/em><\/span>\n<span style=\"color: #0000ff;\"><em> Total&nbsp;&nbsp;&nbsp;&nbsp;<\/em><\/span>\n<em><span style=\"color: #0000ff;\">Point estimates and 95 % CIs:<\/span><\/em>\n<em><span style=\"color: #0000ff;\"> ---------------------------------------------------------<\/span><\/em>\n<em><span style=\"color: #0000ff;\"> Inc risk ratio&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <\/span><\/em>\n<em><span style=\"color: #0000ff;\"> Odds ratio&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <\/span><\/em>\n<em><span style=\"color: #0000ff;\"> Attrib risk *&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <\/span><\/em>\n<em><span style=\"color: #0000ff;\"> Attrib risk in population *&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <\/span><\/em>\n<em><span style=\"color: #0000ff;\"> Attrib fraction in exposed (%)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <\/span><\/em>\n<em><span style=\"color: #0000ff;\"> Attrib fraction in population (%)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <\/span><\/em>\n<em><span style=\"color: #0000ff;\"> ---------------------------------------------------------<\/span><\/em>\n<em><span style=\"color: #0000ff;\"> * Cases per 100 population units<\/span><\/em><span style=\"color: #0000ff;\"><em>&nbsp;&nbsp;<\/em><\/span><\/code><\/pre>\n\n\n\n<p>The package epiR<sup class='sup-ref-note' id='note-zotero-ref-p338-r2-o1'><a class='sup-ref-note' href='#zotero-ref-p338-r2'>2<\/a><\/sup> should be <a href=\"https:\/\/pcool.dyndns.org\/index.php\/packages\/\" data-type=\"page\" data-id=\"22\">installed<\/a>.<\/p>\n\n\n\n<p class=\"is-style-text-annotation is-style-text-annotation--1\">Please note, the maximum likelihood odds ratio is a different estimate than the cross product ratio. <\/p>\n\n\n\n<pre class=\"wp-block-code has-small-font-size\"><code><span style=\"color: #ff0000;\">summary(epi.tests(mat))<\/span>\n<span style=\"color: #0000ff;\">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; est&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; lower&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; upper<\/span>\n<span style=\"color: #0000ff;\">aprev&nbsp;&nbsp;&nbsp;&nbsp; <\/span>\n<span style=\"color: #0000ff;\">tprev&nbsp;&nbsp;&nbsp;&nbsp; <\/span>\n<span style=\"color: #0000ff;\">se&nbsp;&nbsp;&nbsp;&nbsp; <\/span>\n<span style=\"color: #0000ff;\">sp&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <\/span>\n<span style=\"color: #0000ff;\">diag.acc&nbsp; <\/span>\n<span style=\"color: #0000ff;\">diag.or&nbsp;<span style=\"color: #000000;\"> {This is the cross product (diagnostic) odds ratio}<\/span><\/span>\n<span style=\"color: #0000ff;\">nnd&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <\/span>\n<span style=\"color: #0000ff;\">youden&nbsp;&nbsp; <\/span>\n<span style=\"color: #0000ff;\">ppv&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <\/span>\n<span style=\"color: #0000ff;\">npv&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <\/span>\n<span style=\"color: #0000ff;\">plr&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <\/span>\n<span style=\"color: #0000ff;\">nlr<\/span><\/code><\/pre>\n\n\n\n<p><span style=\"color: #0000ff;\"><br><\/span><\/p>\n\n\n\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Screening is the application of a test to asymptomatic individuals to assess the likelihood that an individual has a particular disease. Surveillance of disease is used to look for changes in trends or distribution of the disease. Prevalence ~ Incidence x Mean disease duration (when population and disorder stable) A risk factor is an exposure [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"inline_featured_image":false,"footnotes":""},"class_list":["post-338","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/pcool.dyndns.org\/index.php\/wp-json\/wp\/v2\/pages\/338","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pcool.dyndns.org\/index.php\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/pcool.dyndns.org\/index.php\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/pcool.dyndns.org\/index.php\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/pcool.dyndns.org\/index.php\/wp-json\/wp\/v2\/comments?post=338"}],"version-history":[{"count":5,"href":"https:\/\/pcool.dyndns.org\/index.php\/wp-json\/wp\/v2\/pages\/338\/revisions"}],"predecessor-version":[{"id":4491,"href":"https:\/\/pcool.dyndns.org\/index.php\/wp-json\/wp\/v2\/pages\/338\/revisions\/4491"}],"wp:attachment":[{"href":"https:\/\/pcool.dyndns.org\/index.php\/wp-json\/wp\/v2\/media?parent=338"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}