* 示例任务：供用户在新会话中运行，由 Stata 执行，不读取个人研究文件。
* 已核对 Stata 19.5；输入 auto，输出两组回归与比较表；没有外部依赖。
* Marx Max example: compare two linear regressions using Stata's bundled auto data.
* Input: sysuse auto. No user research files or external packages required.
* Output: regression output and a model comparison in the Results window/log.
* Sample: complete cases on price, weight, mpg. Both models use robust SEs.
* Run in a fresh practice session: clear replaces data currently in memory.
* No random operation or seed. No causal interpretation is implied.
sysuse auto, clear
describe price weight mpg
summarize price weight mpg
generate byte sample = !missing(price, weight, mpg)
regress price weight if sample, vce(robust)
estimates store baseline
regress price weight mpg if sample, vce(robust)
estimates store extended
estimates table baseline extended, b(%9.3f) se(%9.3f) stats(N r2)