Why Sample Size (and Randomization) Matter in A/B Testing for Fundraising

As nonprofit marketers, A/B testing feels like the ultimate tool to optimize your acquisition and fundraising campaigns. Want better email open rates? Test your subject lines. Looking to increase average gift sizes? Experiment with your appeal ask amounts and impact messaging. But while testing sounds simple, two key factors — sample size and randomization — can impact your insights and make or break your results.

Without enough data or proper randomization, you risk drawing incorrect conclusions, wasting resources, and missing opportunities to engage donors effectively. Let’s dive into why these two factors matter and how to get them right.

Small Sample Sizes, Big Problems

Imagine spending hours (or days!) crafting two email variations only to find inconclusive results. Many nonprofit marketing and fundraising teams enthusiastically embrace A/B testing of their communications and fundraising campaigns, only to find themselves disappointed with inconclusive results.

The most common culprit isn’t poor strategy or flawed execution – it’s insufficient sample size.

When your test groups are too small, you’re essentially setting yourself up for failure before you even begin. With insufficient data, differences between your groups might just be random noise rather than meaningful patterns.

The sample size really does matter because it directly impacts your ability to detect meaningful differences between testing groups. Running tests with smaller samples is like trying to judge the quality of a book by reading a single page – it might give you a glimpse, but it’s not enough to understand the whole story.

So how much data do you really need?

Let’s break down two common testing scenarios:

Response Rate Optimization

– Current response rate: 10%

– Desired improvement: 10% (to reach 11%)

– Required sample size: 7,248 per group

– Total campaign size needed: 14,500

Gift Amount Enhancement

– Current average gift: $100

– Desired improvement: 10% (to reach $110)

– Required sample size: 8,306 per group

– Total campaign size needed: 16,700

True Randomization: Why It Matters

Even if your sample size is sufficient, your results can still be skewed without proper randomization. Here’s why:

Randomization ensures fairness. Proper randomization guarantees each donor has an equal chance of being assigned to either group. This prevents biases, like one group accidentally containing more frequent donors.

Avoiding skewed results. If you assign test groups manually or without randomization tools, the outcomes might reflect your list’s structure (e.g., alphabetic, geographic) rather than the true effect of your test.

Getting statistically reliable insights. Randomization ensures the test results reflect reality, not anomalies in your data.

Think of randomization as the foundation of a strong bridge. Even with the right materials (sample size), the bridge won’t hold if the foundation is uneven. Proper randomization ensures your test groups are balanced and unbiased, allowing you to trust that your results reflect reality and not random quirks in your data.

Tools for Proper Randomization

Using built-in A/B testing tools in platforms like email marketing software (e.g., Mailchimp, HubSpot, or Campaign Monitor) or donor databases can help you randomize lists automatically. These tools:

• Randomly assign donors to groups without manual intervention.
• Track and compare results within the same platform.
• Eliminate human error, ensuring cleaner, more reliable data.

Avoid assigning supporters manually or simply splitting a list alphabetically as these methods often lead to skewed test groups.

Sometimes you or your fundraising team will need to run A/B tests outside of your marketing campaign software. In these instances use a true list randomization tool like Dataro’s FREE A/B Test Calculator to allocate donors or supporters to particular tests to ensure true randomization. This way you control for any biasses and know the outcomes of your test are valid.

Summary: Best Practices for A/B Testing

A/B testing can be a game-changer for nonprofit marketing and fundraising campaigns. But success requires a thoughtful approach and the right tools. From planning your sample size to randomizing your lists and effectively tracking your results, following best practices ensures your tests deliver reliable insights and actionable results.

Here’s how to set yourself up for success and make the most of your testing efforts.

Plan Your Test & Calculate Your Sample Size

Use online calculators to plan your test before you launch and determine how many donors you need for a statistically valid test. Factor in your current metrics and desired improvements, and plan your campaign timeline accordingly.

Randomize Your Groups

Use tools or software that automatically randomize assignments to ensure unbiased results (like Dataro’s A/B Randomizer Tool or email platform features).

Set Realistic Goals

Test for achievable improvements based on historical data and be prepared to invest the necessary time and resources required to produce reliable results. Prioritize tests that align with your team capacity and where you can realistically achieve actionable outcomes without overextending your team or budget.

Track and Adjust

Monitor your test, document your methodology and processes, and refine your campaigns based on results achieved.  Be prepared to run tests for longer periods if needed.

The Takeaway

For nonprofit marketers and fundraisers, A/B testing isn’t just about running experiments, it’s about learning what works for your donors and building better campaigns. But success depends on getting the fundamentals right: enough data and true randomization.

Before launching your next A/B test, ask yourself:

• Do we have access to enough donors for a statistically valid test?
• Are our improvement goals realistic given our sample size constraints?
• Do we have the tools and infrastructure to properly track and analyze the results?

By ensuring your A/B tests are valid and unbiased and are set up to deliver actionable insights, you can optimize your marketing and fundraising campaigns with confidence and turn every insight into meaningful action for your mission.

Ready to elevate your testing game? Start with a sample size calculator or use Dataro’s FREE A/B Test Randomizer tool.