Private tutoring has become popular throughout the world. However, evidence for the effect of private tutoring on students' academic outcome is inconclusive; therefore, this paper presents an alternative framework: a nonparametric bounds method. The present examination uses, for the first time, a large representative data-set in a European setting to identify the causal effect of self-initiated private tutoring. Under relatively weak assumptions, I find some evidence that private tutoring improves students' outcome in reading. However, the results indicate a heterogeneous and nonlinear effect of private tutoring, e.g. a threshold may exist after which private tutoring becomes ineffective or even detrimental.