Various methods have been shown to compute the Speech Transmission Index (STI) using speech as a probe stimulus (Goldsworthy & Greenberg, J. Acoust. Soc. Am., 116, 3679-3689, 2004). Frequency-domain methods, while accurate at predicting the long-term STI, cannot predict short-term changes due to fluctuating backgrounds. Time-domain methods also work well on long speech segments and have the added potential to be used for short-time analysis. This study investigates the accuracy of two time-domain STI methods: envelope regression (ER) and normalized correlation (NC), as functions of window length, in various acoustically degraded environments with multiple talkers and speaking styles. Short-time STIs are compared with a short-time Theoretical STI, derived from octave-band signal-to-noise ratios and reverberation times. For windows as short as 0.3 s, the ER and NC Methods track the short-time Theoretical STI and both the Theoretical and ER Methods converge to the long-term result for windows greater than 4 s. Short-time STIs are also compared to intelligibility measurements on clear/conversational speech. Correlations between STI and intelligibility scores are high at the sentence and word levels and, consistent with the scores, short-time methods predict a higher average value of STI for clear than for conversational speech.