Implementation Strategy and Solution Design
Start from questions, not variables
An implementation succeeds when the Solution Design Reference (SDR) answers business questions in measurable terms before anyone touches code. Work top-down: business objective → KPI → metric (event) → dimension (eVar or prop) → data layer field → tag rule. Each KPI needs an owner, a definition, and an example value so QA has something objective to test against.
Know the variable taxonomy cold. Success events are counter, numeric, or currency based. eVars are conversion-level dimensions with an allocation setting (recent, original, linear) and an expiration setting (hit, visit, event, custom days). Props are hit-level traffic variables used for pathing and list props. List vars hold multiple delimited values on one hit. Report suite strategy matters just as much: a global suite plus regional or brand suites means every hit is multiplied, so agree on multi-suite tagging, currency, time zone, and base URL up front.
Traps that appear in questions: choosing a prop when allocation and persistence are required; ignoring the prop value length limit and losing long IDs; assuming two report suites can be merged later (they can only be virtualized or exported); designing an eVar without deciding expiration, which silently changes year-over-year numbers; and skipping a naming convention, so the SDR drifts within a quarter.
Rule of thumb: for every new KPI, write the SDR row plus one sentence stating what decision it will change. If nobody can name the decision, do not instrument it.
Worked example: a B2B site wants whitepaper downloads by campaign. Download becomes a counter event with no serialization because repeat downloads matter. Campaign becomes an eVar with original allocation and a 30-day expiration. File name becomes a hit-expiring eVar. A prop on the download page supports pathing. The SDR records the data layer fields (for example download.fileName and campaign.trackingCode) so the tag rule maps context data rather than scraping the DOM.