In data transmission and storage systems, combined error correction and detection procedures are often used to provide high reliability. This paper considers the use of separate concatenated codes

and

for error correction and detection, respectively. It examines the error detection performance of code

to determine how the probability of undetected error depends on the choice of

and

. A comparison is made of the probability of undetected error achievable by code

with and without error correction, respectively. Asymptotic bounds for low bit error rates are developed which provide criteria to be considered when choosing codes

and

.