Effort versus Ability

What should a teacher say to a student who has accomplished a task; "You're good at this", or "You've been working hard"? This question has been addressed decades ago by Dale Schunk. Which of these two should be used depends on timing. The helpful feedback is the one that is deemed credible by the student at that specific moment. "Working hard" or effort feedback is not useful when the task a student successfully accomplishes is seen by that student as easy. Praising effort when effort is not really obvious implies that the student lacks ability. Effort feedback works when a student has indeed struggled. On the other hand, "You're good" or ability feedback is not good when the student is still at the beginning stage of acquiring or learning, and has not really established a skill. It is not credible.

How a student views effort and ability also changes with age. Young children often cannot distinguish between the two, but by the time they finish elementary school, most have developed an inverse relationship between the two, that is, high effort is equated to low ability, and low effort is regarded as identical to high ability. This has been illustrated in various studies. An example is from Folmer and coworkers:

Nicholls' effort/ability level (derived for self and other) as a function of age.
Copied from 
J Exp Child Psychol. Author manuscript; available in PMC 2009 Feb 1. Published in final edited form as: J Exp Child Psychol. 2008 Feb; 99(2): 114–134.Published online 2007 Dec 11.doi: 10.1016/j.jecp.2007.09.003

The effort/ability level in the above scale corresponds to the following:
Level 1 (age 5-6), effort and ability are not differentiated and their relation to outcome is unclear; 
Level 2 (age 7-9), children attribute outcome purely to effort 
Level 3 (age 10-11), children begin to distinguish between ability and effort and will inconsistently attribute outcome to one or the other 
Level 4 (age 12 and up), ability is seen as a factor that limits the effect of effort.
At the college level, a recent study attempts to find the reason why students believe in an inverse relation between effort and ability. The article scheduled to be published in the Journal of Educational Psychology finds that the perceived reason behind the effort matters. If the effort is perceived to be elicited by difficulty of the task then students readily equate high ability to low effort. On the other hand, if the effort is perceived to be arising from self-motivation then the inverse relation between effort and ability does not always hold.

Giving feedback really requires knowledge of a student's perspective. It needs to be aware of the process. It needs to be a function of time and understanding where a student currently stands. Choosing between "You're good" or "You have worked hard" really requires knowing a student. Otherwise, the feedback may not make sense and therefore not helpful.