Fundamentals

Trend

Trend is the slow drift in level — up, down, or flattening — after you look past the weekly wiggle. Growth, decay, saturation. It is the part of the series that would still be there if you blurred out the season.

A new app's daily actives can rise for eighteen months and then bend. A factory's scrap rate can creep down after a process change. If you fit only last week's pattern, you will miss the bend until it is expensive.

Trends end. Linear-forever is a comfortable lie. When a series is saturating, a model that loved last year's slope will overshoot. Watch for a changing slope, not only a pretty line.

De-trending is optional. Some models want a stationary leftover. Others (and most foundation models) will eat trend as-is. You still want to see it with your eyes, because a trend you did not notice will be credited to a promo you did.