1. Introduction
aging population is increasing rapidly in both developed and developing countries. The percentage of the world’s aging population (≥60 years of age) was 8% in 1950 and is estimated to reach 21% in 2050 [1]. Therefore, it is necessary to pay close attention to this age group. This age group mostly suffers from psychological and physical problems. Psychologists have devised a variety of measures for tackling these problems including disappointment and depression. However, after the emergence of positive psychology, scholars, instead of focusing on a negative experience, deal with positive constructs such as happiness [2 ,3 ,4]. Understanding happiness, which is influenced by the culture and conditions of each community, has been largely based on the studies conducted in western countries and does not apply to the countries with cultural diversity in Asia. Therefore, due to the lack of a suitable happiness scale for Iranian older adults, the present study aims to design a questionnaire for measuring happiness in the elderly according to Iranian culture and determine its psychometric properties.
2. Materials and Methods 
The present study was methodological research. It was conducted in four phases recommended by Schneider et al. (2004) [7]. In the first step of the first stage, the study was based on the content analysis of qualitative research aimed to achieve the concept of happiness in the elderly. The data were saturated and codified in the semi-structured interviews with 19 participants include 9 elderlies, 3 family members, 2 nurses, 2 psychologists, 1 social worker, and 2 physicians. 
In the second phase, to achieve the factors of the elderly happiness questionnaire, the factors of happiness were extracted based on the review of texts, measurement tools, and interviews with the participants. A questionnaire was created, using the results of the qualitative research implemented in the first phase, related to the inductions from the expert panel. 
In the third phase, the validity of the measure was checked. Firstly, the face validity of the measure was examined using the qualitative method according to the opinions of 10 older adults. Secondly, the quantitative face validity was examined according to the opinions of 10 elderly persons. The assessment was based on the item impact method. Thirdly, content validity was examined using the qualitative and quantitative methods. 
In the qualitative phase, the opinions of 5 experts were received to observe grammar, wording, item allocation, and scaling [6]. Then, the quantitative content validity was examined using Content Validity Ratio (CVR) and Content Validity Index (CVI; Scale-CVI/Ave) [7], according to 20 experts.
In the third step, a sample of 304 old people aged 60 or over, of both sexes was selected for exploratory factor analysis. In the fourth phase, the reliability of the measure was checked with the internal consistency and stability. 
To calculate internal consistency of the items, Cronbach’s alpha coefficient of statements was calculated. The test-retest method was used to determine the stability of the tool and the obtained scores were compared using ICC.
3. Results 
In one step of our research, which was performed through qualitative content analysis by a semi-structured in-depth interview with 19 participants, the results led to identify the content domain within five dimensions including human rights, enjoyable life, good morals, health, and spirituality. The initial 77 item questionnaire was designed using the results of the first and second phases of the study.
In the assessment of face validity based on the qualitative method, none of the statements was omitted. But in the assessment of face validity based on impact score, 76 statements gained at least 1.5 scores or more, one of them were omitted. In the qualitative content validity method 16 statements, in CVR of the instrument, 4 statements with a score less than 0.62, were omitted. Besides, researchers attempted to make a correct, reasonable, and clear writing of statements based on experts’ view to make face validity for the instrument. Also, in the assessment of CVI, the other 3 statements with a score less than 0.79 were omitted too, and the statements of the questionnaire were reduced to 39. In this way, a Scale-level Content Validity Index of 0.95 for the instrument was obtained. 
The results of construct validity using exploratory factor analysis with varimax rotation indicated the ultimate measure containing 39 questions with five factors (human rights, enjoyable life, good morals, health, and spirituality) with the ability to explain 70.496% of the total elderly happiness variance. The results of the construct validity using known groups' comparisons showed that this questionnaire could successfully discriminate between subgroups of elderly people in the indexes of gender and education status.
Finally, the reliability based on internal consistency and test-retest method indicated [8] that the internal consistency (Cronbach’s alpha) was 0.838–0.950. The test-retest reliability (ICC) of the questionnaire with an interval time of two weeks was 0.95 (P<0.001).
4. Conclusion
The study used both quantitative and qualitative methods to design and determin psychometric properties of the Elderly Happiness Questionnaire in Iran. This questionnaire in the present study was designed based on the native understanding of elderly happiness in the context of Iranian culture alongside the relevant texts. This native understanding in the objectification and benchmarking of the external experience has led to the elderly happiness questionnaire including 39 statements and 5 factors that have  good validity and reliability (internal consistency and stability) in community-dwelling older samples. Thus, the questionnaire can be recommended for use as a trait-measure in Iranian older adults. It can be used among older adults in different settings such as clinical settings, homes, and research environments by health care providers, psychologists, and researchers.

Compliance with ethical guidelines
This article has complied with the principles of ethics in the research, including informed consent of participants, the confidentiality of participants' information, the full explanation of research goals, responding or not responding the questions, and social and cultural considerations.

Funding
This study was extracted from the PhD. dissertation of first author at Department of Health Psychology, Islamic Azad University of Kish International Branch. Also, The Research Vice Chancellor of Islamic Azad University, Kish International Branch has supported this article.

Authors' contributions
All authors equally contributed in preparing this paper.

Conflicts of interest
The authors declared no conflict of interest.

Acknowledgements
We appreciate the Vice Chancellor for Research, Islamic Azad University, Kish International Branch.


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