Table 2. Grade rules of trigger word frequency.

Identifier	Feature abstract layer	Grade	Define
101	TWFL	hp	Fr ≥ 1
102	TWFL	a	Fr> 1 and Fr≤ 5
103	TWFL	b	Fr> 5 and Fr≤ 10
104	TWFL	c	Fr> 10 and Fr≤ 20
105	TWFL	d	Fr> 20 and Fr≤ 30
106	TWFL	e	Fr> 30 and Fr≤ 40
107	TWFL	f	Fr> 40 and Fr≤ 50
108	TWFL	g	Fr> 50

Note: Fr denotes Frequency.