<story><title>Court rules drug dog barely more accurate than a coin flip is good enough</title><url>https://www.washingtonpost.com/news/the-watch/wp/2015/08/04/federal-appeals-court-drug-dog-thats-barely-more-accurate-than-a-coin-flip-is-good-enough/</url></story><parent_chain><item><author>harryh</author><text>This article, and in particular the title, is horrible and shows no understanding of the math in play. The facts do not indicate that the dog &amp;quot;is barely more accurate than a coinflip&amp;quot; if 59% of the indications in question turn out to actually have drugs.&lt;p&gt;Out of 1000 samples the dog is positively indicating for 930 of the (93% of the time) of which 549 are true positives and 381 are false positives. Let&amp;#x27;s be generous and say that all 70 remaining are true negatives.&lt;p&gt;If the dog was subbed out for a coin flip we would have a positive indicator 500 times of which 274 would be true positives and 226 would be false positives. We would then have 275 false negatives and 225 true negatives.&lt;p&gt;So subbing in the dog for a coin catches 275 drug carriers at the expense of an extra 155 false positives. We can argue about whether that is good enough but comparing it to a coinflip makes no sense.</text></item></parent_chain><comment><author>elif</author><text>When i was moving and literally everything i owned and cared about was pulled out of my car at 2am and strewn on the sidewalk in the rain for hours, the &amp;quot;extra 155 false positives&amp;quot; seemed to matter a little more to me than it does to you.&lt;p&gt;This isn&amp;#x27;t a clinical study where the stats are everything. This is a real world situation where people are being profiled and it is legalized.</text></comment>
<story><title>Court rules drug dog barely more accurate than a coin flip is good enough</title><url>https://www.washingtonpost.com/news/the-watch/wp/2015/08/04/federal-appeals-court-drug-dog-thats-barely-more-accurate-than-a-coin-flip-is-good-enough/</url></story><parent_chain><item><author>harryh</author><text>This article, and in particular the title, is horrible and shows no understanding of the math in play. The facts do not indicate that the dog &amp;quot;is barely more accurate than a coinflip&amp;quot; if 59% of the indications in question turn out to actually have drugs.&lt;p&gt;Out of 1000 samples the dog is positively indicating for 930 of the (93% of the time) of which 549 are true positives and 381 are false positives. Let&amp;#x27;s be generous and say that all 70 remaining are true negatives.&lt;p&gt;If the dog was subbed out for a coin flip we would have a positive indicator 500 times of which 274 would be true positives and 226 would be false positives. We would then have 275 false negatives and 225 true negatives.&lt;p&gt;So subbing in the dog for a coin catches 275 drug carriers at the expense of an extra 155 false positives. We can argue about whether that is good enough but comparing it to a coinflip makes no sense.</text></item></parent_chain><comment><author>rayiner</author><text>The article&amp;#x27;s whole analysis is fucked up because it ignores the prior probability.&lt;p&gt;&amp;gt; In U.S. v. Bentley, we see just how damaging the Harris decision really was. Lex, the drug dog that searched Bentley’s car, had a 93 percent alert rate. That is, when Lex was called to search a car, he alerted 93 percent of the time. He was basically a probable cause generator. His success rate was much lower, at 59 percent.&lt;p&gt;In other words, drugs were found in &lt;i&gt;55%&lt;/i&gt; of cars in which he was brought in.[1] The base rate for drug possession in a car is almost certainly a tiny fraction of that. So between the human judgment to bring in Lex, and Lex&amp;#x27;s sniffing, the police are able to find drugs at probabilities that are multiples of the base rate. In contrast, if you searched cars based on a coin flip (i.e. randomly), you&amp;#x27;d expect to find drugs exactly at the base rate.&lt;p&gt;[1] Being successful &amp;gt; 50% of the time is pretty much the definition of probable cause.</text></comment>