ASTR 430 ยท Lecture 06

Lecture 06: Model Selection and Residuals

Research Methods in Astronomy

A lower residual sum is not automatically a better scientific model when flexibility and residual structure are ignored.

Objectives and route

Terms for This Lecture

Residual

Definition: The signed difference between an observation and a model prediction.

Why it matters here: In Model Selection and Residuals, Residual is needed to reason about weighted least squares, residuals, AIC/BIC, and posterior checks rather than merely label the phenomenon.

Relationship: It is interpreted alongside Weighted least squares and the lecture relation below; identify which quantities are observed, calibrated, or model-derived.

Limitation or common confusion: BIC does not replace residual diagnosis or scientific interpretation.

\(BIC=k ln(n)-2 ln(L_max)\)

Why the method matters

A lower residual sum is not automatically a better scientific model when flexibility and residual structure are ignored.

Keep observation, calibration, transformation, model, and claim separate so each can be challenged.

residual structure
weighted least squares, residuals, AIC/BIC, and posterior checks. The geometry distinguishes inputs, transformations, and inferential limits.

Formal object

\(BIC=k ln(n)-2 ln(L_max)\)

Define fields, units, defaults, version identifiers, and the boundary between measured, adopted, and inferred quantities.

Read the evidence

Identify which part is input, transformation, and claim. A caption should let a reader reproduce the path without asking the author what was omitted.

residual structure
weighted least squares, residuals, AIC/BIC, and posterior checks. The geometry distinguishes inputs, transformations, and inferential limits.

Worked audit

BIC_2=23.9915, BIC_4=23.9829; delta BIC=0.00853545.

BIC does not replace residual diagnosis or scientific interpretation.

Estimate direction first, record units or decision criteria, then test one assumption. This check is generated from shared ASTR430 values.

Failure modes

Active reasoning

Prompt: For n=20 compare two-parameter chi-square 18 with four-parameter chi-square 12.

What would you record so a skeptical collaborator could rerun or challenge the conclusion?

Paired workshop

The workshop turns this method into a reproducible artifact: a query manifest, screening log, uncertainty table, figure critique, time budget, or proposal review. The deliverable includes result and provenance.

Takeaways

Exact source anchors

Astropy Modeling and Statistics documentation; NASA Exoplanet Archive documentation.

Archive, software, and ethics citations with verification status are in reference-log.md.