Table 3. Association between Internet addiction and sleep quality stratified by sample characteristic variables (n=768)

Control confounding variables	Difference in stratified OR (p)	Stratum-specific OR	Crude OR	Difference between stratum-specific OR and crude OR by %	Confounding/ Interaction
Gender	0.071	3.52	3.69	4.60	No
Grade level	0.820	3.73	3.69	1.08	No
Academic performance	0.436	3.59	3.69	2.71	No
Conduct	0.388	3.70	3.69	0.27	No
Living with someone	0.375	3.68	3.69	0.27	No
Family financial status	0.168	3.67	3.69	0.54	No
Having extra classes	0.570	3.59	3.69	2.71	No
School satisfaction	0.453	3.43	3.69	7.04	No
Academic pressure	0.322	3.09	3.69	16.2	Confounding
Using the Internet after 9pm	0.100	3.19	3.69	13.5	Confounding
Degree of Internet usage	0.771	3.51	3.69	4.87	No
Internet usage time	0.848	3.5	3.69	5.14	No
Degree of alcohol usage	0.392	3.57	3.69	3.25	No
Smoking	0.716	3.65	3.69	1.08	No
Social media	0.497	3.57	3.69	3.25	No
Playing games	0.361	3.68	3.69	0.27	No
Reading news	0.171	3.69	3.69	0.00	No
For learning purpose	0.523	3.74	3.69	1.35	No
Watching film	0.060	3.61	3.69	2.16	No
Shopping	0.884	3.77	3.69	2.16	No