Experiments, MVPs and validated learning

Discovery produces hypotheses: commuters will check a space indicator before leaving home; an alert prompt after the first schedule view will raise sign-ups; tourists will read a first-visit guide. Each is an assumption, and every untested assumption is a bet with the client's money. This lesson covers how product teams test assumptions cheaply before and after building: minimum viable products, prototypes and fake doors before writing much code, and controlled experiments such as A/B tests once a product has users. The goal, in Eric Ries's phrase from The Lean Startup, is validated learning: evidence that a hypothesis is true or false, gathered as cheaply and quickly as possible.

Build, measure, learn

The lean startup loop is build, measure, learn: build the smallest thing that tests a hypothesis, measure what happens, and learn whether to persevere, change direction (pivot) or stop. The loop is usually planned backwards. First decide what you need to learn, then what you need to measure to learn it, and only then what you need to build to measure it. Many experiments need no software at all. A clear hypothesis has a specific form: We believe that [change] for [users] will result in [outcome]; we will know we are right when [measurable signal] within [time].

For the live space indicator, Team Lumen wrote: We believe that showing remaining vehicle space for the next two sailings will cause weekday commuters to check before leaving home; we will know we are right when at least 40 per cent of surveyed commuters report checking it at least three times in a two-week pilot, and early arrivals in the lineup fall. The threshold is set before the experiment, not after. Deciding success criteria in advance is what separates an experiment from a demonstration.

Minimum viable products

A minimum viable product (MVP) is the smallest thing that lets you learn whether an idea is worth pursuing. The term is widely misused to mean a buggy first release. Ries's meaning is narrower: an MVP is a learning tool, and it may not be a product at all. The useful question is not what is the smallest product we can ship, but what is the cheapest way to test our riskiest assumption.

Some MVP types raise ethical questions. A fake door that misleads people can damage trust, so it should explain honestly on click that the feature is being considered and let people register interest; it should never take payment for something that does not exist. A Wizard of Oz test should avoid processing personal information in ways users did not expect. And the concierge approach, though unscalable, is often the fastest way to learn what users really want, because the team interacts with each one personally.

Prototyping and usability testing

Before building an interface, test it. Usability testing asks representative users to complete realistic tasks with a prototype while thinking aloud, and the team watches where they hesitate, misunderstand or fail. Five users per round find most major usability problems, per Jakob Nielsen's widely cited rule of thumb; several small rounds with fixes in between beat one large round. For Driftwood, Team Lumen tested three tasks: find the next sailing to the north island; check whether today's 5:40 sailing is delayed; find out whether the island terminal accepts cash. The third task failed for four of five users, because the answer was buried in a frequently asked questions page.

A/B tests

Once a product has enough users, the strongest evidence comes from a controlled experiment, usually called an A/B test. Users are randomly assigned to a control (A) or a variant (B), the team measures a primary metric such as the delay-alert sign-up rate, and statistics determine whether any difference is larger than chance would produce. Randomization is the key: it balances everything else, such as time of day, device and type of traveller, between groups, so a difference in the metric can be attributed to the change.

A test needs a sample size decided in advance. It depends on the baseline rate, the minimum detectable effect (MDE), meaning the smallest lift worth detecting, the significance level alpha (the false positive rate, usually 5 per cent), and power (the chance of detecting a real effect of size MDE, usually 80 per cent). Small effects need large samples, roughly in proportion to one over the square of the effect. The code computes visitors per arm for a 20 per cent baseline.

# Sample size per arm for a two-proportion A/B test (normal approximation).
from statistics import NormalDist
def n_per_arm(p1, mde, alpha=0.05, power=0.80):
    p2 = p1 + mde
    z_a = NormalDist().inv_cdf(1 - alpha / 2)
    z_b = NormalDist().inv_cdf(power)
    pbar = (p1 + p2) / 2
    num = (z_a * (2 * pbar * (1 - pbar)) ** 0.5
           + z_b * (p1 * (1 - p1) + p2 * (1 - p2)) ** 0.5) ** 2
    return int(num / mde ** 2) + 1
base = 0.20   # 20% of visitors currently sign up for delay alerts (illustrative)
for mde in (0.01, 0.02, 0.04, 0.08):
    print(f"detect +{mde:.0%} absolute lift: {n_per_arm(base, mde):6d} visitors per arm")

detect +1% absolute lift:  25583 visitors per arm
detect +2% absolute lift:   6510 visitors per arm
detect +4% absolute lift:   1683 visitors per arm
detect +8% absolute lift:    447 visitors per arm

Driftwood's site gets perhaps 4,000 visitors a week in summer and far fewer in winter. Detecting a 1 percentage-point lift would take months; detecting 4 points takes under a week. Low-traffic products, which include most client projects, can only detect large effects with A/B tests and should rely more on qualitative methods. This is an important reality check: A/B testing is powerful for large consumer products, but often impractical for small clients.

When the test finishes, analyse it once with the planned test. The code below runs a two-proportion z-test on invented results: the variant shows the sign-up prompt after a traveller first views the schedule.

# Evaluate a finished test with a two-proportion z-test and a confidence interval.
from statistics import NormalDist
a_users, a_conv = 2100, 420    # control: plain sign-up link
b_users, b_conv = 2080, 491    # variant: sign-up prompt after first schedule view
pa, pb = a_conv / a_users, b_conv / b_users
p = (a_conv + b_conv) / (a_users + b_users)
se_pool = (p * (1 - p) * (1 / a_users + 1 / b_users)) ** 0.5
z = (pb - pa) / se_pool
p_value = 2 * (1 - NormalDist().cdf(abs(z)))
se = (pa * (1 - pa) / a_users + pb * (1 - pb) / b_users) ** 0.5
lo, hi = (pb - pa) - 1.96 * se, (pb - pa) + 1.96 * se
print(f"control {pa:.1%}  variant {pb:.1%}  lift {pb - pa:+.1%}")
print(f"z = {z:.2f}, p = {p_value:.4f}")
print(f"95% CI for lift: {lo:+.1%} to {hi:+.1%}")

control 20.0%  variant 23.6%  lift +3.6%
z = 2.82, p = 0.0048
95% CI for lift: +1.1% to +6.1%

The p-value is the probability of seeing a difference at least this large if there were truly no difference. It is not the probability that the variant is better, and a p-value below 0.05 does not guarantee the effect is large or important. The confidence interval is more informative: it gives a range of plausible lifts. Here the whole interval is above zero, so the prompt very likely helps, and the plausible size of the effect is roughly 1 to 6 points. For a fuller treatment of hypothesis testing see a statistics course such as STAT-151; here we focus on how to run experiments well as a product team.

The peeking problem

The most common A/B testing mistake is peeking: checking results repeatedly while the test runs and stopping as soon as the result is significant. Each look is another chance for random noise to cross the threshold, so the real false positive rate rises far above 5 per cent. The simulation runs A/A tests, where both groups get the same experience so any significant result is a false positive, and stops at the first look with p below 0.05.

# Why peeking inflates false positives: an A/A test (no real difference),
# checked after every batch of visitors and stopped at the first p < 0.05.
import random
from statistics import NormalDist
random.seed(401)
def p_value(ca, na, cb, nb):
    p = (ca + cb) / (na + nb)
    se = (p * (1 - p) * (1 / na + 1 / nb)) ** 0.5
    if se == 0:
        return 1.0
    z = ((cb / nb) - (ca / na)) / se
    return 2 * (1 - NormalDist().cdf(abs(z)))
def run(peeks, batch=200, rate=0.20):
    ca = cb = na = nb = 0
    for k in range(peeks):
        ca += sum(random.random() < rate for _ in range(batch)); na += batch
        cb += sum(random.random() < rate for _ in range(batch)); nb += batch
        if peeks > 1 and p_value(ca, na, cb, nb) < 0.05:
            return True
    return p_value(ca, na, cb, nb) < 0.05
trials = 1000
for peeks in (1, 5, 10, 20):
    fp = sum(run(peeks) for _ in range(trials))
    print(f"{peeks:2d} looks: false positive rate {fp / trials:.1%}")

 1 looks: false positive rate 5.4%
 5 looks: false positive rate 14.1%
10 looks: false positive rate 20.6%
20 looks: false positive rate 26.6%

Remedies exist. The simplest is discipline: fix the sample size or duration in advance and analyse once. Sequential testing methods, used by many commercial experimentation platforms, adjust the threshold so that repeated looks keep the overall error rate at 5 per cent. Bayesian approaches report the probability of improvement but have their own stopping pitfalls. Whatever method is used, the team should pre-register the primary metric, the sample size and the decision rule.

Other experiment pitfalls

Several other mistakes are common. Testing many metrics and reporting whichever moved (multiple comparisons) produces false discoveries; pick one primary metric and a few guardrail metrics, such as page load time and unsubscribe rate, that must not get worse. Novelty effects make new designs look better at first simply because they are new; run tests for at least a full weekly cycle. Sample ratio mismatch, where the split is noticeably different from 50/50, signals a bug in assignment and invalidates the result. And tests on a few days of summer traffic may not generalize to winter commuters.

Pivot or persevere

Experiments feed decisions. If evidence supports the hypothesis, persevere and invest. If it contradicts the hypothesis, consider a pivot: a structured change in strategy, such as targeting a different segment (truck drivers rather than tourists), solving a different problem (delay information rather than space), or changing the solution (alerts rather than a live indicator). Driftwood's Wizard of Oz pilot found that commuters checked the indicator but found the deckhand's manual updates too infrequent to trust; the team persevered with the goal but pivoted the solution to an SMS alert when the deck reached 90 per cent full, which was feasible with manual counts.

Worked examples