All stats, even traditional ones (even offensive ones, even pitching ones), needs about three years of data to tell us what the players true talent level is. To create a good 'mean'. Doesn't mean we toss out a players performance for one year and say it didn't happen, just means that the way he performed that one year, either offensively, defensively or hurling the ball, may not be what his true talent is. One year defensive metrics can be used to gauge how a player did that year and defensive performance, just like offense and pitching, can fluctuate. The worst thing Fangraphs did when explaining defensive metrics was explain the obvious (that obvious thing being that more data gives us a player's true talent level). They should have just reminded people that it's the same situation with all stats, not just defensive metrics. They do say one years worth of defensive data can tell us what the player did that year (particularly DRS, which is why I prefer to use that). They talk about 1 year and 3 year samples as being stable, meaning one year to tell us what the player did for that year (yearly performance) and 3 years to get a mean (measure true talent level). 'The other thing to remember is that DRS isn’t going to work well in small sample sizes, especially a couple of months or less. Once you get to one and three-year samples, it’s a relatively solid metric but defensive itself is quite variable so you need a good amount of data for the metrics to become particularly useful.' Right there it says one year. Having said all that, defensive metrics will get better (but still better than eye test or errors/fielding %), just like better offensive metrics have been created (like wRC+) that are better than what we used to have.