NEW YORK, Nov 20 - The Federal Reserve Bank of Chicago said on Thursday its National Activity Index for October slipped due mainly to a weak reading of industrial production. The index came in at 0.01 in October, from a downwardly revised 0.16 in September. Factory production ran below trend in October, and employment continued to drag on the index, despite recent reports of improvement in the job market. The personal consumption and housing components provided the bright spot for the month, lifting the overall index 0.10 but not enough to overcome the negative contributions. The three-month moving average of the index suffered a steeper decline, to -0.07 in October from 0.06 in September.//