Measurement of Socioeconomic Status in Iran: A Systematic Review

Aims: The socioeconomic status is considered to be a multi-dimensional indicator, and there is no definite standard to measure it.The aim of the present study is to perform a systematic review to evaluate whether there are the same indicators for measuring socioeconomic status in the published articles in the medical sociology in Iran. Study Design: Systematic Review. Methodology: We searched the English language literature on socioeconomic status in health researches in Iran between 1990 and October 2014: MEDLINE using PubMed, Scopus, Web of Science, and Science Direct. In addition, we searched these Iranian electronic databases: Iranian Scientific Information Database (SID) and IranMedex. Results: Our finding indicated the three factors – education, occupation, and income – were the dominant indicators for measuring socioeconomic status in both English and Persian articles. Only four studies (English articles) had composite measure and made a standard indicator in their Review Article Mahdavian and Safizadeh; AJAEES, 6(1): 1-15, 2015; Article no.AJAEES.2015.057 2 studies. Eleven studies had no clear categories for measuring socioeconomic indicators. Conclusion: We encountered various definitions and indicators, mostly because of poor quality of the studies with diversity in methods and populations.


INTRODUCTION
The socioeconomic status is important and determines health risk factors in every society. It also has effect on the actual access and utilization of various available health facilities [1]. The socioeconomic status (SES) is considered to be a complicated and multi-dimensional indicator, and there is no definite standard to measure it. The first constituent of the socioeconomic status includes the social class of people, which indicates whether a person has an acceptable social position or not. Generally, education is considered as a representative of a social class of people. The educational level is measured with different scales, such as binary scale (literate / illiterate), numeric scale (years of education), and highest educational level (categorical). The other aspects of the social class of people are much more complex to be approach [2]. The second constituent of SES covers affluence and proceeds of people; however, this component has no clear definition.
The indicators, such as income, property, and possessions of the individual or the household, might also be measured [3]. SES was identified by Mueller and Parcel in 1981 as an individual's or group's within a hierarchal social structure based on their access to wealth, prestige, and power [4].
The members of society vary in the way of their usual access to jobs, assets, income, and power. Despite the fact that the impacts of SES on health are well known, the measurement of SES in health studies has several methodological and analytical issues [2]. One of them is lack of accuracy and reliability of measures. In addition, gathering individual SES data (e.g., high proportion of non-response for income variables) and categorizing of children, women, the retired, and unemployed can be conducted with difficulties. Finally, Weak correlation between individual SES measure among some groups (i.e., income, education, and occupation) leads to the inaccurate results.
Regarding to the impact of factors on the health status and health disparities, socioeconomic status implicatesas a frequent contributor in some population [5]. To put it simply, differences in health status of individuals with different socioeconomic statusinfluence on the health. For example, a low socioeconomic status raises cardiovascular risk of US population, in European countries cancer mainly associated withlow level of socioeconomic status, however [6].
Various studies have defined several indicators for measuring socioeconomic status. Selecting the best variables and approaches for assessing SES should be dependent on consideration of the relevance of the measurements for the target groups and the outcomes under study. There is no doubt that there are several factors for defining the socioeconomic status, which is variable from one study to another [7][8][9]. Even though the number of studies has explained about socioeconomic status indicators, no systematic review was available in the literature of medical sociology to signify how various it is and which indicators are applicable.The systematic Review studies are considered as the strongest and the most valid type of medical studies [10]. For the considerable amount of data we encounter,one of the best methods for policy makers is systematic review [11].
The aim of the present study is to perform a systematic review to evaluate whether there are the same indicators for measuring socioeconomic status in the published articles in the medical sociology in Iran.

Data Sources / Search Strategy
In order to review the English language literature on socioeconomic status in health researches in Iran between 1990 and December 2014, we searched the following English electronic databases: MEDLINE using PubMed, Scopus, Web of Science, and Science Direct. In addition, we searched these Iranian electronic databases with Persian language: Iranian Scientific Information Database (SID), and IranMedex. The searches of articles were undertaken between Dec30 and 31, 2014. The Authors also scrutinized reference lists of included articles (no articles were added). For additional articles, we conducted the supplementary search in Google and Google scholar(no articles were added).

Search Terms and Strategy
We searched these English terms (MESH terms) and their corresponding Persian equivalents: "Socioeconomic Status", "Status, Socioeconomic", "Socioeconomic Factor", "Factors, and Socioeconomic", and "Factor, Socioeconomic". Each of these words was combined with "OR" and then combined, using "AND", with Iran OR Iranian OR Farsi OR Persian.

Inclusion / Exclusion Criteria
The authors considered all types of original studies (on adults and children), i.e., clinical trials, longitudinal, cohort, case-control, ecology, systematic review, and cross-sectional studies. Letters, reports, conference papers, organizational reports, opinions, or editorial papers were excluded. All studies addressed socioeconomic status whether it was the main subject or not. The studies did not mention clear socioeconomic definitions and categories were excluded.

Selection, Reading, and Information Extraction
In the next step, all citations that reported the socioeconomic status were reviewed. One author independently was selected and reviewed the articles by following these stages; inclusion and exclusion criteria were assessed both in reading the titles and the abstracts of search results. The author read to determine the final articles, and the duplicated articles were eliminated. After that, the data extraction tables were completed for each article using these characteristics: sample size, unit of observations, study design / measurement tool, category of socioeconomic status, definitions of socioeconomic measures, first author, year, and also language. Then, we found all full-texts of the articles selected, and the exclusion criteria were also applied to the fulltexts.

Search Results
The database search identified 309 articles in English and Persian. Primarily, 257 studies were identified in English and Persian biomedical databases respectively after eliminating the duplicated articles (Fig. 1). In the next step, of the 249 studies, 180 were of no relevance to the current review according to their abstracts and titles. In the full text evaluation step, 25 studies did not have distinct socioeconomic definitions and categories; they were omitted. Finally, 46 articles that had the criteria of the study were selected.

Socioeconomic Status Definitions and Indicators
All studies provided their definitions for socioeconomic status, but they used different definitions and categories. The most common indicators of socioeconomic status were education (63.04%), Occupation (45.65%), and Income (28.26%) (Fig. 2).

Composite measurements and scales
In order to construct socioeconomic status, four articles used composite measures (Table 2). One article used principal component analysis with three variables: women's occupation, her husband's occupation, and family's income. Then, persons were categorized into low, intermediate, and high socioeconomic statuses [12]. One other study used multiple correspondence analyses and created wealth score based on the appliance ownership variables [13]. The third study used social class with the scores obtained from parents' occupational status, parents' educational level, and family's income [14] Another study, for measuring socioeconomic status, computed parents' education and their occupational status using the four-factor Hollingshead index based on the Hollingshead criteria [15].

Education, occupation, and income
Four studies (8.7%) used education, occupation, and income for the socioeconomic status definitions [28][29][30][31] (Table 3). Two of which classified education into different number categories (four and seven) [28,30]. In the reviewed articles, two of them mentioned no specific criteria for measuring education [29,31].
Similarly about occupation, one of the articles divided it into six categories [30]. Furthermore, three studies had no clear definition for measuring occupation [29,31]. For measuring income, one study chose the Iranian central bank indictor [28]; another study classified it into six categories based on the Iranian currency (Rial) [30]. Two studies did not use clear definition for this index [29,31].

Living place, education, occupation, and income
Three studies measured socioeconomic status based on living place, education, occupation, and income [38][39][40] (Table 1). Living place in one article was not available [39]. Two studies used living place based on the rural and urban residence [38,40]. . For education, two studies divided education into four and three categories [38,39]. One study measured the years of education [40]. Occupation indicator had different number categories (six, five, and two) [38][39][40].
For measuring income, only one study divided it into four categories based on Rial [39]. One study measured the income as economic status based on the three categories: good, fair, weak [38]. In one study, the income as economic status was not available [40].

Education and income
Two studies measured income and education as socioeconomic indicators [41,42] (Table 3). One article divided education and income into four categories [42]. Another study used four categories for educational level and income based on the poverty level [41].

Income
Two articles used only income for socioeconomic status [43,44] (Table 1).Among them, one study used monthly income [44]. Another study calculated this indicator base on dollar [43].

Discussion
All of the studies mentioned that socioeconomic status is contingent on various indicators, which means that there is lack of a gold standard in health literature in Iran. We found that results indicated the three factors -education, occupation and income-were the dominant and frequent indicators for measuring socioeconomic status in both English and Persian articles [28][29][30][31]. Only four studies (English articles) had composite measure and made a standard indicator in their studies [12][13][14][15].
It is similar to the Duncan American SES Scale which is classified occupation according to education and income [58]. Nam and Power [59] also defined the SES indicator based on their occupation status score education and family income for person in a family. In addition, Green (1970)composite a measure based on income, education and occupation [60]. Hodge employed the household prestige which was meant the participations in a survey rated the social position of households described in terms of spouses' occupations, income, and ethnicities [61]. Although ethnicity is important as a SES parameter in other countries [62], there is no evidence in our study about ethnicity variable as a socioeconomic status indicator in Iran. This is mainly because of the ethnicity in health literature in Iran doesn't consider as part of SES measure.
Socioeconomic status,regardless of evaluated by education, occupation, or income, is mostly associated with the various ranges of health disparities. As some studies have revealed that low or middle socioeconomic status linked to the cancer, cardiovascular diseases, and higher mortality rate [6,63], however we did not find any evidence in our literature.
Also, British researchers used the data from survey respondents who are asked to identify the occupations of four friends. The occupational rankings of the respondent and friends were analyzed with multidimensional scaling techniques which yield an ordinal Cambridge Scale score for each participation [64]. Similarly, the National Statistics Socioeconomic Classification applied occupational relationship and relied on employment theory and grouped persons into, typically, eight nominal strata [65]. Furthermore, we encountered that our findings provided no information about these two scales. Finally, our findings indicated that some articles(eleven studies) had no clear categories for measuring socioeconomic indicators [47][48][49][50][51][52][53][54][55][56][57].

CONCLUSION
This study was done with the aim of assessing the definition and the categories of socioeconomic status in health literature in Iran based on published studies. We encountered various definitions and indicators, mainly because of poor quality of articles with diversity in socioeconomic parameter. As it was mentioned before, the measurement of SES is difficult since it is a multi-dimensional indicator. Measuring this indicator in the most researches assessed without standard indicators, we have to measure socioeconomic status, nevertheless. Finally,for solving discrepancies between the SES definitions, we are proposing to design standard indicators that are suitable for health literature in Iran.