![\"firstError\"](\"http:\/\/www.eversholt.org.uk\/blog\/wp-content\/uploads\/2014\/02\/firstError.png\")
<\/a><\/p>\nIn the chart, the part breaks at time A. But I don’t recognise there is a problem, because I’ve not yet noticed anything wrong. As far as I am concerned, the car is still working fine. The blue line on the chart represents this period. Eventually, though, the broken part causes an error that I can detect. In my case, the engine stopped when I expected it to be rotating. The red line represents this event, which occurred at time B, some time after time A.<\/p>\n
I then realise that the car has a fault. However, I don’t get it fixed straight away, so the car goes on being faulty for some while – the horizontal part of the red line. I can still get the car to start, and drive it around – carefully – but I know it is broken.<\/p>\n
In order to know that there is a fault, I need to be able to detect an error, and I won’t be able to do that until the fault is exercised<\/strong> to produce an error I can notice. A fault which isn’t exercised doesn’t produce any errors.<\/p>\n A car headlamp bulb is a good example of a fault that’s not detected until it’s exercised. The bulb can break any time, when the lamp is on or off. When the headlights are switched off, even if there is a broken bulb, there is no error. I’ll not know until I switch on the lights – of course, when it’s dark and I really need it. That’s when I exercise the fault to produce an error, the lamp not producing light when I expect it to.<\/p>\n","protected":false},"excerpt":{"rendered":" Let’s consider a system – say, my car – that works perfectly when it is new. It has no design flaws. It goes on working for some considerable time, and then some part breaks. The car now has a problem, a fault, that should be repaired by replacing the broken part. But, I don’t yet […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[7],"tags":[],"class_list":["post-154","post","type-post","status-publish","format-standard","hentry","category-fixed-yet"],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.eversholt.org.uk\/blog\/wp-json\/wp\/v2\/posts\/154","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.eversholt.org.uk\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.eversholt.org.uk\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.eversholt.org.uk\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.eversholt.org.uk\/blog\/wp-json\/wp\/v2\/comments?post=154"}],"version-history":[{"count":6,"href":"https:\/\/www.eversholt.org.uk\/blog\/wp-json\/wp\/v2\/posts\/154\/revisions"}],"predecessor-version":[{"id":161,"href":"https:\/\/www.eversholt.org.uk\/blog\/wp-json\/wp\/v2\/posts\/154\/revisions\/161"}],"wp:attachment":[{"href":"https:\/\/www.eversholt.org.uk\/blog\/wp-json\/wp\/v2\/media?parent=154"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.eversholt.org.uk\/blog\/wp-json\/wp\/v2\/categories?post=154"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.eversholt.org.uk\/blog\/wp-json\/wp\/v2\/tags?post=154"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}