What you will learn
- Look-ahead
- Survivorship
- Parameter search
- Repeated holdout use
Look-ahead
Lookahead occurs when a decision uses information unavailable at its timestamp. It can enter through final bar values, revised releases, future pivots or dataset-wide transformations. Audit availability, not merely the date label attached to a row.
Survivorship
Survivorship and selection arise when the sample contains only entities or strategies that remained available or attractive. Retain the inclusion process and rejected variants so the observed result is not mistaken for an unselected experiment.
Parameter search
Preprocessing can leak information when scaling, imputation or feature selection is fitted on evaluation data. Fit learned transformations on the allowed training portion and apply them to later data under the frozen procedure.
Repeated holdout use
Repeatedly checking an evaluation set to choose changes makes it part of development. Label that use honestly and obtain separate evidence for the selected final procedure rather than continuing to call the reused set untouched.
Worked example
A scaler uses the mean of the entire dataset, including future evaluation observations. Even though the prediction model is fitted only on earlier rows, its inputs have already incorporated later information.
Try it yourself
Specify a corrected transformation pipeline and explain how to apply it during each temporal evaluation fold.
Show the worked solution
Fit the scaler and other learned preprocessing only on each fold's training portion, freeze those fitted objects and transform the corresponding later evaluation portion. Repeat the complete fitting process inside each fold without using its evaluation targets or distribution to choose the transformation.
Apply this to your course project
Create a preregistered validation protocol and report rejected variants.
Keep the calculation inputs, assumptions and decisions with your work. Practical exercises are self-reviewed; the scored knowledge checks assess the questions shown, not an independent certification of practical competence.