Google Analytics\u2019<\/a> Site Speed reports<\/a> are pretty darn great. They report automatically on various milestones in the process the browser undertakes when rendering content. These reports leverage the Navigation Timing API<\/a> of the web browser, and they are (typically) collected on the first Page View hit of a page.<\/p>\n And this is all fine. As I said, it\u2019s a great feature of Google Analytics, and lends itself handily to spotting issues in the quite complex client-server negotiation that goes on when your web browser requests content from the web server.<\/p>\n <\/p>\n <\/a><\/p>\n<\/div>\n However, there\u2019s a glitch. These automatic page timings are collected once per tracker instance per page<\/em>, meaning if you have multiple trackers on the page, each set to collect site speed samples, you might inadvertently send the same page speed data multiple times<\/strong>. This will naturally inflate the numbers that Google Analytics reports on, and your data will be ruined, as deduplication is really difficult.<\/p>\n <\/p>\n <\/a><\/p>\n<\/div>\n So if you have any of the following in place, you might be in the risk group:<\/p>\n Single-page site where you send \u201cvirtual\u201d Page Views to Google Analytics<\/p>\n<\/li>\n Multiple trackers on the page, each collecting to the same Universal Analytics property (UA-XXXXX-Y)<\/p>\n<\/li>\n Manually set site speed sample rate (e.g. at 100%)<\/p>\n<\/li>\n Google Tag Manager with a Page View Tag that has multiple Triggers attached to it<\/p>\n<\/li>\n<\/ol>\n If any of these rang true, you might have a data quality issue that should be fixed immediately.<\/p>\n \n \n Subscribe to the Simmer newsletter<\/a> to get the latest news and content from Simo Ahava into your email inbox!\n <\/p>\n <\/span><\/p>\n By default<\/a>, 1% of Page View hits are sampled for page timings. So if you have the following code on the site:<\/p>\n There\u2019s a 1\/100 chance that upon the first time command is executed, this particular request will grab the available Navigation Timing data and send it to Google Analytics. And, again, this is fine. That\u2019s what we want. Now if you have the following code:<\/p>\n That\u2019s two separate Page View hits firing on the same page, you\u2019re still fine. Since they use the same tracker object (the default tracker<\/em>), even if you win the odds and manage to hit the 1\/100 twice on the same page, the page timing data is only sent once, since Universal Analytics only sends timing data once per tracker per page.<\/p>\n However, if you have this:<\/p>\n If both these trackers collect to the same Google Analytics property (UA-XXXXXX-Y), and if both trackers manage to somehow make the 1\/100 cut, you will be sending the same page timing data twice<\/em>, just for different page paths. And that will warp your data!<\/p>\n It gets worse\u2026<\/p>\n Google Tag Manager<\/a>, for all the good it does, has one complication that makes a lot of things difficult: it assigns a unique tracker object for every single tag that fires – even if it\u2019s the same tag firing multiple times.<\/p>\n Take another look at the image at the beginning of this article. There you have a Page View Tag which has the Now, here\u2019s the issue: because it has Every single time this Page View Tag fires, it will send the same page timing data to Google Analytics!<\/strong><\/p>\n That\u2019s multiplication on a grand level! It\u2019s even more confusing since the same<\/em> page timing data is sent to multiple page paths, making deduplication in the reports really difficult to do.<\/p>\n Here are some obvious fixes to this issue.<\/p>\n This is pretty easy if you\u2019re using the Universal Analytics snippet. Set This would ensure that only a single set of page timings is sent per page.<\/p>\n In GTM, by default, this would be impossible, because each tag has its own unique tracker name.<\/p>\n This is irrelevant in Universal Analytics, as only the first pageview per tracker is sampled anyway. In Google Tag Manager, however, this might just fix the issue for you.<\/p>\n For this to work, you need to fire the first Page View hit of a page using a unique trigger. Typically, you\u2019d have the \u201cAll Pages\u201d Trigger on the Page View Tag, and then another Trigger for the custom Page View. Thus, a single-page app might have the following Tag catering for all Page Views:<\/p>\n <\/p>\n <\/a><\/p>\n<\/div>\n Now, this Tag will<\/em> be problematic if left like this, since the Page Timing sample will be sent every single time the Tag fires – once for the All Pages Trigger, and each time the Custom Event Trigger fires.<\/p>\n To fix this, use this Custom JavaScript Variable as the value of the So your Tag looks like this:<\/p>\n <\/p>\n <\/a><\/p>\n<\/div>\n This JavaScript returns In other words, only the All Pages Trigger will send the site speed sample, ensuring it is only sent once on the page.<\/p>\n If you don\u2019t have the means to distinguish the first Page View from all the subsequent triggerings, you could also add hitCallback<\/a> to your Page View Tag, which sets a global flag (or even a This is the most risky but also the most effective way to fix it. Remember how GTM uses a unique tracker name for every instance of a Tag? Well you can always set the Tracker Name<\/strong> field with some value to make sure that every time the Tag fires it uses the same tracker object. Thus the page timing sample is only sent once. To do this, you\u2019d have to configure one of the settings in the Page View Tag:<\/p>\n![\"Collected](\"https:\/\/www.simoahava.com\/images\/2016\/10\/hit-to-ga.jpg#ZgotmplZ\")
<\/p>\n![\"Site](\"https:\/\/www.simoahava.com\/images\/2016\/10\/site-speed-sample-rate-pageview.jpg#ZgotmplZ\")
<\/p>\n\n
\n X<\/span><\/p>\n
\n The Simmer Newsletter<\/strong>\n <\/p>\nFraming the problem<\/h2>\n
ga('send'<\/span>, 'pageview'<\/span>);\nga('send'<\/span>, 'pageview'<\/span>, {page: '\/some-custom-page\/'<\/span>});\n<\/code><\/pre>\n<\/div>\nga('send'<\/span>, 'pageview'<\/span>);\nga('secondTracker.send'<\/span>, 'pageview'<\/span>, {page: '\/some-custom-page\/'<\/span>});\n<\/code><\/pre>\n<\/div>\nThe problem with Google Tag Manager<\/h2>\n
siteSpeedSampleRate<\/code> set to 100%, which means that every single time the Page View hit is sent, it should be used to sample page timing data. It also has a Trigger which fires whenever a Custom Event is pushed into dataLayer<\/code>. So this is your typical \u201cvirtual\u201d Page View (man I hate that term – all Page Views are virtual!) Tag, set to fire with a custom page path whenever the event triggers.<\/p>\nsiteSpeedSampleRate<\/code> set to 100, and because it will fire multiple times on the page, and because each time it fires it will have a unique tracker name:<\/p>\nFixes<\/h2>\n
1. Only sample a single tracker<\/h3>\n
siteSpeedSampleRate<\/code> to a proper value in only one tracker, and set all other trackers to 0.<\/p>\nga('create'<\/span>, 'UA-12345-1'<\/span>, {siteSpeedSampleRate: 100<\/span>});\nga('create'<\/span>, 'UA-12345-1'<\/span>, {name: 'secondTracker'<\/span>, siteSpeedSampleRate: 0<\/span>});\n<\/code><\/pre>\n<\/div>\n2. Only sample the first pageview<\/h3>\n
![\"Custom](\"https:\/\/www.simoahava.com\/images\/2016\/10\/custom-page-view.jpg#ZgotmplZ\")
<\/p>\nsiteSpeedSampleRate<\/code> field:<\/p>\nfunction<\/span>() {\n return<\/span> {{Event}} === 'gtm.js'<\/span> ? 100<\/span> : 0<\/span>;\n}\n<\/code><\/pre>\n<\/div>\n![\"Site](\"https:\/\/www.simoahava.com\/images\/2016\/10\/site-speed-javascript.jpg#ZgotmplZ\")
<\/p>\n100<\/code> for the All Pages Event (that\u2019s the gtm.js<\/strong> event name), and 0<\/code> for all other Triggers.<\/p>\ndataLayer<\/code> variable) to true<\/code>, indicating that the tag has fired (at least) once. Then you can use this flag as a condition for the siteSpeedSampleRate<\/code> field, only sending the 100<\/code> value if this flag is false.<\/p>\n3. Set the Tracker Name in your GTM Tag<\/h3>\n
![\"Tracker](\"https:\/\/www.simoahava.com\/images\/2016\/10\/tracker-name-gtm.jpg#ZgotmplZ\"){kind=tracking}
<\/p>\n