Ivan Avdouevski Riitta Kerminen Jarmo Makkonen Ari Visa Missing values in user activity classification applications utilizing wireless sensors. 151-155 2008 MOBIWAC http://doi.acm.org/10.1145/1454659.1454688 conf/mobiwac/2008 db/conf/mobiwac/mobiwac2008.html#AvdouevskiKMV08