Many user-end applications require an estimate of the quality of coded video or images without having access to the original, i.e. a no-reference quality metric. Furthermore, in many such applications, the compressed video bitstream is also not available. The paper describes methods for using the statistical properties of the coded video data to estimate the quantization error caused by compression without accessing either the original pictures or the bitstream. A commonly used quality metric, the peak signal-to-noise ratio (PSNR) is subsequently computed from the estimated quantization error. Since quantization error is the most significant loss incurred during typical coding schemes, the estimated PSNR, or any PSNR-based quality metric may be used to gauge the overall quality of the pictures.