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I chose rules and statistical methods for my health app because the right architecture depends on the job the software must do—not on whether a technology is fashionable. For a bounded task with established guidance, explicit rules may be easier to inspect and evaluate. When formal guidance is missing, statistical prediction may help find patterns in data. An LLM can generate fluent responses, but fluency alone does not show that a health answer is correct or beneficial.

The available account does not specify the app’s purpose, users, implementation, clinical claims, testing, or the author’s actual reasons for choosing its architecture. Those details matter: without them, no one can responsibly claim that this particular choice improved safety, accuracy, cost, or outcomes. The case is best understood through the engineering questions every health-app team should answer before selecting an approach.

Start with the job the app has to do

“Health app” is not a sufficiently precise technical or regulatory description. A symptom explainer, a medication reminder, a risk estimate, and a treatment recommendation have different inputs, outputs, and consequences if they are wrong. Before choosing rules, statistical prediction, or an LLM, define the function narrowly.

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  • Input: What information does the app receive, and how reliable and representative is it?
  • Output: Does it calculate a value, flag a condition, rank options, or generate individualized advice?
  • User: Is the output for a patient, a caregiver, or a healthcare professional?
  • Error: What could happen if the result is wrong, incomplete, delayed, or misunderstood?
  • Evidence: What supports the behavior, and how will the team test it in the setting where it will be used?

These questions are more useful than asking whether one technology is inherently safest. Rules can encode incomplete or outdated guidance; statistical systems can fail when data or deployment conditions differ from those used to develop them; and generated text can be persuasive without being reliable.

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What rules and statistical methods are good at

Rules for bounded tasks with established guidance

Rules encode specified behavior as fixed logic—for example, an if-then condition tied to an established threshold or guideline. The Office of the National Coordinator for Health Information Technology (ONC) describes evidence-based rules as grounded in expert consensus informed by clinical trials, observational studies, and other research. When a task has accepted guidance and the app needs to apply it consistently, fixed rules can make the path from input to output comparatively straightforward to inspect.

That does not make rules automatically correct. The team still has to justify the chosen guidance, handle exceptions and missing inputs, keep the logic current, and test whether it behaves appropriately for intended users. A rule that is faithfully implemented can still encode a poor or unsuitable decision.

Statistical prediction where formal guidance is absent

Statistical and machine-learning approaches can identify patterns in real-world data and may help address questions for which formal guidelines do not exist. Their usefulness depends on the data and context: poor-quality or unrepresentative source data can undermine performance, and a model may not behave as expected when the people, workflow, or setting at deployment differ from those represented in training data. Some models are also difficult to interpret.

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ONC contrasts these data-derived predictions with fixed, evidence-based rules; it does not establish that one approach is best for every health task. The relevant question is whether the data support the particular prediction the app intends to make—and whether the team can detect failures in its intended setting. ONC’s explainer on rules and predictive interventions was published on 19 October 2022.

What an LLM changes—and what it does not solve

An LLM can produce natural-language responses, but convincing wording is not evidence of clinical correctness. On 16 May 2023, the World Health Organization warned that LLM responses can appear authoritative and plausible while being incorrect or seriously flawed, particularly in health contexts. WHO also identified risks involving bias, protection of sensitive data, consent, and the need for transparency, human supervision, rigorous evaluation, and clear evidence of benefit before widespread routine use.

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Those concerns do not prove that every LLM health application is unsafe or that a rules-and-statistics system is safer. They do mean a team should test the actual system and task, rather than treating a fluent answer as a successful one. An LLM also does not remove the need to decide what information the product may use, what it should say when uncertain, how a person can seek help, or who is accountable for monitoring failures. WHO’s statement on safe and ethical AI for health sets out these cautions.

Compare approaches against the same requirements

A useful architecture decision compares candidate approaches on the app’s intended behavior and evidence—not on labels such as “AI” or “deterministic.” The table describes general trade-offs, not measured results for this particular app.

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Question Rules Statistical prediction LLM
Is there an established evidence base? Can encode accepted guidance as explicit logic; the team must still select and maintain appropriate guidance. Can be useful where patterns in data address a question without formal guidelines. Fluent output does not itself establish a clinical evidence base or benefit.
Can the output be inspected? Fixed conditions and pathways can be reviewed directly. Some models are opaque, making their behavior harder to explain. Generated responses may sound plausible even when flawed; they require rigorous evaluation.
What can undermine performance? Incomplete, outdated, or poorly chosen logic and mishandled exceptions. Poor source data, mismatch between training and deployment contexts, and limited interpretability. Incorrect or seriously flawed health responses, alongside bias and privacy or consent concerns.
What must the team establish? That the encoded guidance and edge-case behavior fit the intended task. That the data and validation reflect the intended users and deployment setting. That evaluation, supervision, transparency, privacy protections, and evidence of benefit are adequate for the intended use.

None of these comparisons supplies an accuracy rate, cost, latency, or outcome result for the app described by the title. Such claims require app-specific measurements and test methods; they cannot be inferred from the general properties of the approaches.

Validate the behavior that users will encounter

Validation should follow the product’s intended use, including its users, inputs, outputs, and consequences of error. For any approach, a team needs to check expected cases as well as missing, unusual, or misleading inputs, and establish how it will notice failures after release. A benchmark that does not reflect the real deployment context cannot establish how the system will behave there.

Monitoring matters because guidance can change, data distributions can shift, and real-world use can reveal failure modes that were not apparent during development. WHO emphasizes rigorous evaluation and human oversight for LLMs in health; ONC’s discussion of data quality and context is also relevant to predictive systems. The sources do not provide a universal test protocol or a result for this app, so a defensible account of the author’s decision would need to say what was tested, against what, and what happened.

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Do not infer FDA status from the app’s label

In the United States, calling something a “health app,” “AI,” or “clinical decision support” does not settle its regulatory status. FDA’s analysis is function-specific and depends on intended use, claims, inputs, outputs, and users; some clinical decision support functions may fall outside the device definition under statutory criteria, while others remain subject to device oversight.

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In its FAQ, FDA states: “Software that is meant to perform simple medical calculations routinely used in clinical practice is not considered Non-Device CDS software.” The FAQ then describes circumstances in which FDA intends to exercise enforcement discretion; that statement should not be generalized to mean all such calculations are treated identically. FDA’s navigator also explains that patient-facing functions do not meet the criterion limited to supporting or recommending options to healthcare professionals. Time-critical decisions and specific preventive, diagnostic, or treatment outputs can also affect whether an exclusion applies. Classification therefore requires the details of the individual software function, not a broad label.

FDA’s generative-AI discussion-paper page addresses risk assessment, premarket evaluation, and postmarket monitoring, but explicitly says the paper “does not represent draft or final guidance.” It is a policy discussion, not binding final guidance or a statement of new requirements. Read the FDA Clinical Decision Support Software FAQ, the FDA clinical decision support navigator, and the FDA generative-AI discussion-paper page for the applicable policy context.

What a credible first-person case study should disclose

The title describes a personal architecture choice, but the available facts do not establish what the author’s app does or why the author made it. A useful engineering account should connect that choice to the actual product rather than claim that one stack is generally superior. To make the reasoning assessable, it should identify:

  • the app’s intended users, function, inputs, and outputs;
  • the evidence or guidance that informed its rules and the task assigned to statistical methods;
  • which alternatives were considered and why they did not fit the defined job;
  • the evaluation method, test cases, intended setting, and observed results;
  • how uncertainty, edge cases, privacy, and consent are handled; and
  • what the team monitors after deployment and how it responds when behavior is wrong.

Without those specifics, it is not possible to conclude that this architecture was more accurate, safer, cheaper, or more effective than an LLM. The defensible general lesson is narrower: choose technology to match the task and evidence, then validate and monitor the system users actually encounter.

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