Should you run A/B tests in the browser or on the server? It's one of the most important technical decisions in experimentation. Here's a clear breakdown to help you choose.
The Quick Answer
For most teams: Start with client-side testing. It's faster to set up, doesn't require engineering resources, and handles 90% of use cases. Only move to server-side when you have specific needs that require it.
How They Work
Client-Side Testing
Changes happen in the browser after the page loads.
Server-Side Testing
Changes happen on the server before the page is sent.
Detailed Comparison
| Factor | Client-Side | Server-Side |
|---|---|---|
| Setup Time | Minutes | Days/Weeks |
| Engineering Required | None | Significant |
| Flicker Risk | Possible (but solvable) | None |
| SEO Impact | None (JS changes) | Possible (different HTML) |
| Performance Impact | Small (script size) | None |
| Visual Editor | Yes | No |
| Backend Changes | No | Yes |
| Cost | Lower | Higher |
When to Use Client-Side
When to Use Server-Side
Common Misconceptions
"Server-side is always better"
Not true. Server-side requires significant engineering investment and is overkill for most visual tests. Start with client-side unless you have specific needs.
"Client-side always causes flicker"
Modern tools have solved this. With proper anti-flicker techniques, client-side testing is virtually flicker-free.
"You need to choose one or the other"
Many teams use both. Client-side for marketing experiments, server-side for product experiments. They complement each other.
Decision Framework
Ask yourself these questions:
Are you testing visual/UI changes?
Yes → Client-side is fine
Do you have engineering resources to spare?
No → Client-side is easier
Do you need to test backend logic?
Yes → Server-side required
Is absolute zero flicker critical?
Yes → Server-side is safer
The Bottom Line
Client-side testing is the right choice for 90% of teams. It's faster to set up, requires no engineering, and handles most use cases. Server-side testing is powerful but comes with significant complexity and cost.
Start with client-side. Prove the value of experimentation. Then invest in server-side when you have specific needs that require it.