Stock-Market Data: Accuracy, Coverage, and Gaps

A stock observation needs a timestamp and a definition

A stock quote is useful only when its timestamp, adjustment status, and coverage match the question being asked. A last trade records a transaction; a bid and ask describe prices currently displayed by participants, and neither proves that every order could execute at that level. Before comparing prices, identify the security, listing, price basis, venue, and observation time.

For company analysis, the same discipline applies to reported numbers. Revenue, earnings, cash flow, and share count need a period, unit, and accounting context. A value copied from a vendor screen without its field definition may not match the company’s filing.

Corporate actions change what a price series represents

A split changes the number of shares and the quoted per-share price without, by itself, changing the holder’s proportional claim. Historical prices may be split-adjusted so that a chart is comparable across the event. Dividend adjustments answer a different question: a total-return series includes distributions under a stated reinvestment convention, while an unadjusted price series does not. Never mix these series in a return calculation without documenting the choice.

Other events can alter the security or its share count: a spin-off, rights issue, merger, special distribution, or conversion may require a mapping between old and new identifiers. Check the vendor’s adjustment method and compare it with the company’s disclosure.

Coverage gaps and vendor differences require reconciliation

Check each provider’s documentation for venue coverage, timestamp conventions, trade-condition handling, symbol mapping, and how halted or inactive securities are represented. Do not assume fields from different feeds are interchangeable. A missing value is not automatically zero, and a blank field should not be filled with the previous observation unless the analysis explicitly treats that value as carried forward. Distinguish “no report,” “not applicable,” “not yet published,” and “feed failure” because each has a different implication.

• Keep the source, field name, unit convention, and retrieval time with each observation.
• Mark missing records explicitly rather than substituting a plausible-looking value.
• Reconcile major price or share-count changes to a disclosed corporate action.

Restatements create a version-history problem

An amended filing or corrected vendor record can change a historical series after the original release. That revised value may be the best current description of the past, but it was not necessarily available to a researcher at the earlier date. For an event study or backtest, retain the publication timestamp and, where possible, the version available at the time.

Do not blend a revised denominator with an unrevised numerator or compare fiscal periods that vendors map differently. When a result depends on a particular accounting line, trace it to the filing and note any reclassification or amendment that affects the comparison.

Maintain a compact audit trail

A useful data-quality note explains which series was used, what was excluded, how corporate actions were handled, and which values remain uncertain. If two feeds disagree, report the disagreement and its likely cause before choosing one; apparent precision does not resolve a coverage mismatch. This is educational information, not individualized investment, tax, legal, or accounting advice.