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Client-Side Vs. Server-SideA/B Testing Tools: What’s The Difference?

By Jason Freely / 7 months ago

Marketers of all stripes are obsessed with instruments.

This obsession has bred detailed lists of growth instruments, SEO instruments, and general online marketing instruments. It is no distinct with us in conversion optimization. We nerd out on testing instruments.

While no optimization application has at any time hinged on which resource you used, there are essential distinctions concerning A/B testing tools—from the figures they use, their price, and far more.

A single detail that is generally possibly forgotten or misunderstood is the distinction concerning client-facet and server-facet testing instruments.

Shopper-facet, server-facet: what’s the distinction?

Here’s the essential distinction concerning client-facet and server-facet testing instruments:

Shopper-facet instruments create a variation of your check page by performing manipulations on your browser by way of intelligent JavaScript. Your net server sends the identical model of the page, the default, as it would commonly do. All the versions are reflected by the visitor’s browser. Given that the variations are not accomplished on your server, and are in its place on the visitor’s browser, they’re known as ‘client-facet instruments.’

Server-facet instruments are distinct in that no modification takes place at the browser degree. When the customer lands on your page, a randomly picked model of your check is despatched straight from your server. Sure, this suggests you have to include your builders in testing, but it also provides you robustness and versatility.

Tim Ash was quoted in a TrustRadius report, summing up the distinction concerning the two varieties of instruments very very well:

Tim Ash from Site Tuners

Tim Ash:

“With server-facet instruments, the content management methods are building in testing, preserving numerous variations of content and serving them up based on segmentation. It delivers management over deployment regardless of browser issues. Shopper-facet instruments are normally less difficult for the marketer. They have WYSIWYG editors and make it effortless to make floor degree variations without needing IT, which is a big discomfort level. It is difficult to use them to restructure the registration route or the checkout, however, simply because you will touch the CMS.”

There’s a 3rd form of resource as very well, closely associated to server-facet abilities. There’s only one company I know that utilizes this method (SiteSpect) so it’s not tremendous common. What they use is some thing known as the proxy server or intercept method.

Fundamentally, the resource acts as a proxy server concerning the visitor’s browser and your server. It controls the web-site content that is offered to the customer, so it gets rid of most of the issues that take place with client-facet instruments, even though retaining the robustness of server-facet instruments.

A/B Examination Planning: How to Establish a Approach that Operates

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Okay, but when must I use which?

As Paras Chopra from VWO reported in a site article, the two are not in opposition. Distinct instruments for distinct demands and ambitions:


Paras Chopra:

“Why are we supporting server-facet testing frameworks? Aren’t they in levels of competition with us? Not really! Our aim with Visible Web page Optimizer is to make A/B and multivariate testing less difficult by getting complex hassles out of the photograph. In simple fact, preferably, we want our people by no means to touch any code (be it HTML, JavaScript, CSS, or PHP). So, VWO is suited for folks who don’t want to rely on builders or IT group for performing even the simplest sort of testing (thereby lengthening the time included in having the check up and working).”

Shopper-facet instruments offer ease of use, quicker execution, and a WYSIWYG editor. You don’t always need to just take up your enhancement team’s time to get experiments working.

That reported, server-facet instruments have positive aspects as very well. An post from Google claimed that server-facet implementations offer far more versatility to do items this sort of as:

  • Operate experiments for internet sites with dynamic content.
  • Examination non-UI variations that nevertheless have an have an affect on on your objective. For illustration, a databases question result set that is returned to a person.
  • Combine Google Analytics experiments with your service (e.g., content management provider).
  • Handle experiments employing your personal optimization system.

In summary, they offer far more versatility. They also have a tendency to be far more strong, and they do away with a handful of of the weaknesses of client-facet instruments. Some of these opportunity weaknesses consist of the flicker influence (which we’ll chat about below), load pace, and versatility.

As Chris Goward of WiderFunnel reported in a TrustRadius report, the JavaScript technique (i.e. client-facet) has a great deal of positive aspects, but also some downsides:

Chris Goward

Chris Goward:

“They’ve solved the biggest challenge, which is the complex obstacles to testing. The downside is load time and web-site pace, and the lack of versatility of what you can manipulate. There are circumstances wherever it will have a flicker or it masses far more than it demands to. Other techniques could possibly make it possible for for far more versatility and pace. I don’t assume we have strike the finest remedy however as an market.”

Pete Koomen, co-founder of Optimizely, summed it up very well:


Pete Koomen:
“The advantage of testing on the client facet is pace and simplicity. You can check a great deal of variations quickly without substantially initial expense. On the other hand, testing on the server facet is the two far more perform and normally far more impressive.”

While, as a further way to seem at it, Nate Weiss (CTO of Conductrics) suggests that the ‘skipping IT’ benefit of client-facet instruments could have a expense for some companies. He points out:


Nate Weiss:

“For companies that have a rigorous QA and acceptance procedure for their net content in general (exterior of testing), the “skipping IT” benefit of client-facet instruments has a expense: it results in being a “back door” wherever the normal acceptance procedures and so on finish up having dismissed, simply because the testing resource doesn’t know about the internal workflows that lead to normal content getting revealed. It is then very challenging, if accomplished at all, to hold these two “moving targets” (variations produced commonly, and variations produced by way of the testing resource) in sync.”

In summary, use distinct instruments for distinct utilizes and ambitions.

The flicker influence

As described previously mentioned, one of the weaknesses of client-facet instruments can be the flicker influence. What is it?

Fundamentally, it’s when, for very tiny amount of money of time, your web site customer sees the default page ahead of the variation masses. It is also acknowledged as the “Flash of Primary Content.”

This normally transpires simply because of an asynchronous script with the testing resource. While it has the advantage of not slowing down the page, this form of script can lead to a flicker influence when in a even though.

This matters simply because the human eye detects, and we transform our behavior when we know we’re section of an experiment. While, there is the shocking (fluke) situation that it aids a check. Acquire this illustration from an Unbounce site article:

As David Hauser, co-founder of Grasshopper, reported in the post:


David Hauser:

“Our A/B testing resource experienced a bug that delayed the $25 activation cost from getting crossed out till a handful of seconds just after the page loaded.

This error finished up generating a substantially more substantial uplift than acquiring it by now crossed out on load, when the bug was preset.

The result now is that the activation cost displays, and then is crossed out just after a handful of seconds.”

Most of the time, however, flicker influence final results in statistically major differences in check final results. That’s entirely affecting the check, and can skew the result. The goal is to decrease the flicker to .0001 seconds so folks won’t see anything at all.

So, I described that flicker influence is really only a challenge with client-facet instruments. What are some strategies, if there are any, that we can mitigate this?

Synchronous vs. asynchronous scripts

Shopper-facet instruments can load scripts in two unique strategies: synchronously and asynchronously. What is the distinction?

Synchronous scripts

Synchronous loading is when a testing tool’s script masses totally ahead of the page commences to load. It phone calls back again to the server and waits for an solution. Set a further way, the scripts are loaded sequentially setting up with the tag.

For illustration, Optimizely utilizes a synchronous snippet to prevent the flicker influence and a balanced CDN system to make absolutely sure the affect on the page is minimized.


Asynchronous scripts

Asynchronous is when some scripts can be loaded concurrently. They load even though the testing script operates, perhaps producing a flicker influence if the initial content masses a break up area ahead of the variation.


You can operate client-facet instruments like VWO and Optimizely possibly way. In general, page loading pace is essential, so asynchronous code is normally most popular. The batched scripts make it possible for the webpages to load quicker, and you can normally prevent the flicker influence with a handful of steps.

As Paras Chopra reported in a GrowthHackers thread:


Paras Chopra:

“Generally with VWO’s asynchronous code, there must not be any flickering. In edge circumstances or in circumstances wherever you have not put asynchronous code in the head of the page, flickering could possibly occur. We’ve produced our asynchronous code in this sort of a way that the initial page is hidden till the options are loaded. (The page gets loaded in parallel and that’s why it is async but it’s not displayed).

With regards to VWO’s (or any other tool’s) synchronous code, I wouldn’t suggest it normally unless there are unique circumstances that can not be preset. Synchronous slows down the page pace.”

Alhan Keser from WiderFunnel also expressed a identical sentiment. Until you are working a significant web-site with tons of site visitors, page pace is of principal relevance:


Alhan Keser:

“Asynchronous loading addresses a separate challenge: aiding with all round web-site pace, not FOOC [flash of initial content]. Given the pace of modern CDNs, snippets loading synchronously must be the least of your worries. But, if you are like our neighbors below in Vancouver, PlentyOfFish, with a bajillion people hitting their web-site at the identical time, you may want to be thoughtful of what and how items load on your webpages.”

Other ideas to prevent flicker influence

A single way to prevent the Flicker Outcome that Optimizely outlined is to include a timeout on asynchronous scripts. Their default placing is one second. This suggests that if the A/B testing script is not loaded in that time, the A/B won’t be executed. If the customer has a gradual connection, that customer won’t be involved in the check.

Some other strategies to prevent the flicker influence:

  • Improve your web-site for pace. Make absolutely sure every little thing, especially the pictures, load fast. Shift your web-site onto a fast net host. Use compression, optimized pictures, and content shipping networks.
  • Acquire your A/B testing resource script out of the tag manager (if you use one, like Google Tag Manager).
  • If you can, do break up URL testing in its place.
  • Shell out awareness to the HTML factors you transform. If you produced variations on the page itself (as opposed to performing break up URL testing), the critical detail to hold in brain is the amount of money of code that was changed. If you changed hundreds of strains, you are considerably far more probably to see a flicker influence as opposed to altering button duplicate. So the scope of variations influences every little thing.

What client-facet instruments do proper

Shopper-facet instruments allow entrepreneurs operate checks properly without the support of builders.

That’s not a tiny accomplishment. Development sources are high priced, in time and cash, so enabling testing impartial of products enhancement is productive. Optimizely put it very well:

“When it’s time to just take motion and make variations to your web-site, there is no substitution for focused complex resources—in most corporations, these sources are difficult to occur by and generally demand waiting around in line.”

Examination quicker

Sean Ellis introduced the notion of substantial-tempo testing as a critical mechanism for growth.

The essential premise is that the quicker and far more productive you can operate checks, the far more of them you can operate, and the quicker you can improve. While their bottlenecks ended up typically at the ideation and organizational levels, quite a few optimizers discover bottlenecks at the enhancement and implementation levels. Shopper-facet instruments employing visible editors address this.

In a prior piece, Andrew Anderson wrote about the relevance of pace in testing instruments. He provides a handful of illustrations of instruments as very well:

Andrew Anderson:

“When considering about pace, there are two distinct elements that matter. The 1st is just after your initial deployment and set-up, how fast can you get a check from concept to execution to dwell? The goal for any application is to get most checks with at least 5 encounters by means of that concept in thirty minutes or a lot less. That is a difficult target to meet up with but it does categorical just how essential it is to prioritize pace in testing and the need for general know-how of CSS, HTML, and JavaScript by the operator.

This suggests that products this sort of as templatized rules, effortless web-site conversation, effortless interface, and effortless navigation are crucial. It also suggests that instruments that need elaborate URL focusing on rules, regular IT deployment, or need guide jQuery set-up must be lowered on your priority scale.

An illustration of a resource that for the most section does this proper is Optimizely even though an illustration of these that doesn’t is SiteSpect.”

Andrew also mentioned that, even though loads of instruments offer effortless-to-use visible interfaces to create therapies, the immediately developed jQuery code is generally very lousy. This final results in cross-browser compatibility problems and slowness. Generally have a developer examine and boost the code—or understand jQuery. It will really support.

Server-facet instruments: when to use?

As I’ve described, a big benefit to client-facet instruments is the pace and ease-of-use for not-tremendous complex folks. So what, then, are the positive aspects of server-facet instruments? Why would companies use them?

As Matt Gershoff, the CEO of Conductrics points out, server-facet instruments have their advantages as very well. Here’s what he experienced to say about them:

Matt Gershoff

Matt Gershoff:

“The major advantages of server facet will be:

  • Larger versatility and management. Use check and focusing on for non-visible testing focusing on (call facilities, CMS methods), and tight integration into marketing tech stack for single workflow (CMS, databases, net servers, and so on).
  • Privateness. Continue to keep all testing/focusing on choices internal.
  • Safety. Lower attack floor.
  • For net apps, decrease the need for ‘heavy’ webpages.”

Server-facet instruments may also be better for cellular web-sites that serve dynamic content simply because, as Nate Weiss, CTO of Conductrics, put it, “the slower connections and a lot less impressive browsers in quite a few phones indicate that client-facet JavaScript techniques are at far more of a drawback on cellular.”

He continues to chat about the applicability to indigenous cellular purposes:


Nate Weiss:

“Some verticals go on to change towards indigenous cellular apps for engagement. If your company has a indigenous cellular app or ideas to in the upcoming, choosing a resource that supports server-facet testing now suggests that you will be ready to commence employing the identical instruments for testing in your apps (with developer involvement).”

An additional matter of less discussions that Nate brought up to me was that, depending on the nature of your web-site and the checks you prepare to operate, it may be far more respectful for your site visitors to use server-facet instruments.

As Nate reported, “consider which is far more essential: your advantage when placing checks up, or your visitor’s person encounter (and affect on their cellular details ideas).”

In addition, server-facet instruments broaden your abilities for optimization outside of net-based-visible presentation problems, in a way. What they do is make it possible for you to assume a tiny distinct about optimization, far more in the direction of optimizing any of your transactional marketing attempts.

With client-facet instruments, the major matter of discussion is around the flicker influence. This is an challenge, but it is not the only, or the major, challenge to be reviewed with instruments. Points like versatility, management, and privacy have an affect on your optimization as substantially or far more than flicker, especially if you just take the precautions to mitigate that threat.

Which form is your resource?

Here’s a chart that quickly categorizes well-liked instruments. Allow us know if we’re lacking any or having anything at all wrong:



I’m simplifying items, but here’s the bird’s eye distinction concerning server-facet and client-facet instruments:

Server-facet instruments render code on the server-degree and mail a randomized model of the page to the viewer with no modification on the visitor’s browser. Shopper-facet instruments mail the identical page, but JavaScript on the client’s browser manipulate the appearance on the two the initial and the variation.

Shopper-facet instruments use possibly synchronous or asynchronous code, the two of which have pros and drawbacks relating to pagespeed, flicker influence, and loading problems.

Shopper-facet instruments are less difficult for entrepreneurs to use, and you can generally get checks up and working quicker without burning enhancement sources. Nevertheless, server-facet instruments offer larger versatility and management. They also safeguard your privacy and stability better, and for net apps, decrease the need for ‘heavy webpages.’

Both equally instruments have their use circumstances and don’t need to be used exclusively. Nevertheless, considerably far more essential than the resource you decide on is the procedure by which you enhance. Don’t overthink it with instruments.