A Review of Common Independent Variables Used in Tobacco Smoking Related Studies

Tobacco is the only legal drug, killing many of tobacco users
when used exactly as intended by its manufacturers. According
to the report of World Health Organization, tobacco use including
smoking and smokeless is currently responsible for about six
million deaths across the world each year [1]. This figure includes
about 600,000 people who may die from the effects of secondhand
smoke. Tobacco smoking has been proved associated with
ill health, disability and death from non-communicable chronic
diseases. However, it also leads to an increased risk of death from
communicable diseases. Thus, tobacco use is still one of the biggest
preventable causes of diseases and premature deaths until now.
Although smoking rates fell in the higher income countries during
the 1970s and 1980s, evidence shows that this trend is levelling
out [2].


Introduction
Tobacco is the only legal drug, killing many of tobacco users when used exactly as intended by its manufacturers. According to the report of World Health Organization, tobacco use including smoking and smokeless is currently responsible for about six million deaths across the world each year [1]. This figure includes about 600,000 people who may die from the effects of secondhand smoke. Tobacco smoking has been proved associated with ill health, disability and death from non-communicable chronic diseases. However, it also leads to an increased risk of death from communicable diseases. Thus, tobacco use is still one of the biggest preventable causes of diseases and premature deaths until now.
Although smoking rates fell in the higher income countries during the 1970s and 1980s, evidence shows that this trend is levelling out [2]. The importance of revealing studying and evaluating the effects of smoking on health is obvious. Herein, the effects of tobacco smoking on specific diseases such as chronic obstructive pulmonary disorders, cardiovascular diseases, respiratory diseases, as well as cancers of lung, oral cavity, larynx, oesophagus, stomach, pancreas, colorectum, bladder and kidney etc. have been found for many years [3].
However, researchers conducted studies using different independent variables for different areas. Most studies used a binary variable to indicate the smoking or smoking cessation [4][5][6][7][8][9][10][11][12][13][14][15][16]. Some researchers further classified the variable into three groups including never smokers, ex-smokers and current smokers [17][18][19][20][21][22][23]. Furthermore, some researchers used the conception of smoking amount like the cigarettes smoked per day to classify the study population into some subgroups when they study the effects of smoking [24][25][26][27][28][29][30][31][32]. Moreover, there also some other forms of independent variables have been used such as duration of smoking cessation [33][34][35][36][37][38], age at starting smoking [39][40][41] and tobacco types [14,15,31,42]. Although there already have been plenty of tobacco smoking related reviews, the independent variables used in the studies of tobacco smoking still have not been systematic reviewed. With the development of the technology and the theory, health care can be implemented in many different ways, such as the semantic Web of Things [43] which promotes the secure semantic smart health care [44] and the facial expression recognition [45] which promotes the smart psychological health management. As a results, the health information has been increasingly complicated and then it is important to choose the reasonable variables for analyses.
Herein, this study will review the independent variables used in previous smoking studies. The regular questions to collect the relating independent variables will be summarized. The appropriate regression models as well as their advantages and disadvantages will be listed. This study may help the researchers of tobacco smoking to choose the appropriate independent variables for their studies. The remaining of this paper is arranged as follows. Section 2, Section 3, Section 4 and Section 5 summarize the studies relating binary variables of smoking, multivariate variables of smoking, smoking amount, and others including duration of smoking cessation, age of starting smoking and tobacco types. Section 6 concludes this study. Section 7 points out some future works.

Questions of Data Collection
In this section, we provide two examples of questionnaire to show the regular questions about smoking, which can be used to collect the data for the independent variables of smoking for analysis.  Table 1 shows the part of smoking in the questionnaire of CHNS.
As shown in Table 1, the researchers can obtain the smoking status including never smoker, current smokers, ex-smokers (questions 1 and 3), smoking amount (cigarettes smoked per day) (question 4), the age of starting smoking (question 2), as well as the duration of smoking cessation (question 5). However, CHNS questionnaire did not provide the definition of smoking. Then, we present another questionnaire of a longitudinal survey which focuses on smoking.

This survey is organized by Chinese Center for Disease Control and
Prevention. The section of smoking in questionnaire of this survey are shown in Table 2. As shown in Table 2, this questionnaire is more detailed. In addition to smoking status (questions 1 and 2), smoking amount (question 5), age of starting smoking (question 3) and duration of smoking cessation (question 8), we can also have the tobacco types smoked (question 5) and the reason for smoking cessation (question 7). How many cigarettes do you smoked per day? -(Skip to the next section) 5 How long ago did you stop smoking? (months) If unknown, just record -99

Smokers versus Never Smokers/Non-Smokers
By separating the volunteers into smokers and never smokers/ non-smokers, many researchers studied the relationship between smoking and specific diseases. Amaral  tomography study [7].

Smoking Amount
This section provides studies review of the smoking amount.

Other Variables
Except smoking status and smoking amount, there are also some other independent variables that can be used in the tobacco smoking related studies such as the duration of smoking cessation, age of starting smoking, as well as tobacco types. This section provides gives the studies review for these variables.

Duration of Smoking Cessation
The

Age of Starting Smoking
The age of starting smoking is usually used to study the long-

Current & Future Developments
Although a lot of smoking-related studies have been carried out, more new forms of independent variables of smoking are needed to conduct smoking-related studies as research of smoking go on. In the future, the independent variable of smoking may be developed for two kinds of research: 1). the research about e-cigarettes; and 2). the research of the quantitative effects of smoking.

Conflict of Interest
There is no conflict of interest for this study.