It is still an open question whether software agents should be personified in the interface. In order to study the effects of faces and facial expressions in the interface, a series of experiments was conducted to compare subjects' responses to and evaluation of different faces and facial expressions. The experimental results obtained demonstrate that: 1) personified interfaces help users engage in a task, and are well suited for an entertainment domain; 2) people's impressions of a face in a task are different from ones of the face in isolation. Perceived intelligence of a face is determined not by the agent's appearance but by its competence; 3) there is a dichotomy between user groups which have opposite opinions about personification. Thus, agent-based interfaces should be flexible to support the diversity of users' preferences and the nature of tasks.

This paper proposes a new evaluation method for ARQ schemes. The decoded bit error rate versus Eb/N0 and the bandwidth expansion ratio are adopted as criteria to measure performance capability. The method makes it possible to compare the performance capabilities of various error control schemes on the same basis. Since this method clarifies the Eb/N0 and the bandwidth required for obtaining the desired BER, it gives some useful indication on deciding which scheme to use in a particular application. FEC, ARQ, Type- hybrid ARQ, and Type- hybrid ARQ, each of which uses a binary BCH code with a common code length, are taken up as examples for analysis. The numerical results show that well designed hybrid ARQ schemes can considerably reduce Eb/N0, at a cost of a small bandwidth increase. Based on the foregoing analysis, this paper also proposes a constant-rate ARQ scheme with finite buffers. This scheme offers a constant throughput rate and a constant delay time as a conventional FEC scheme. It requires less Eb/N0 than an FEC scheme. The numerical results are presented for some examples. The results on the fixed bandwidth expansion ratio show that the proposed scheme with a binary BCH code requires less Eb/N0 than an FEC scheme using a convolutional code and soft decision Viterbi decoding. The decoder of the proposed scheme is much simpler than a Viterbi decoder. This proposed rate-constant ARQ scheme is suitable for application to those systems in which a feedback channel is available and a constant throughput rate and a constant delay time are required.