Iranian Journal of

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Background & Objectives: White spot, as one of the infectious viral diseases, has made severe losses in shrimp ponds all over the world. Despite extensive efforts made to deal with and control the disease, white spot continues to be a major health problem in shrimp farms across Iran. In this work, the significance of the risk factors of white spot disease epidemic occurred in shrimp ponds of Choubdeh farming site in Khuzestan province of Iran is determined.
Methods: A cross sectional study was conducted from June 1, 2010, to September 22, 2010 in 223 shrimp ponds of the site. Data was collected on 17 variables, thought to be associated with the occurrence and epidemic of white spot, with the aid of the shrimp ponds owners and fisheries and veterinary organizations. The occurrence of white spot disease in the farming site was determined by clinical symptoms and the results of conventional PCR tests. the effectiveness of the risk factors was established by odds ratio (OR).
Results: It is found that poor management of birds fighting (OR=3.72), less educated farm foreman
(OR= 3.29) and poor filtration of the intake water (OR= 3.43) are significantly affected the occurrence of the disease while little changes in the salinity of shrimp ponds (OR= 0.16) decreases the odds of the disease.
Conclusion: These findings help better develop shrimp farming across Iran, especially in Khuzestan province. 

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Sampling hidden populations is challenging due to the lack of convenience statistical frames. Since most populations exposed to special diseases are hidden and hard to reach, sampling methods that produce representative and efficient samples from the populations have become a study subject for researches all over the world. Because of the unknown probability of selecting samples in conventional sampling methods and also invalidity of generalizing the results of non-probability sampling methods to the statistical population, the necessity of introducing probability chain-referral sampling methods, such as the respondent driven sampling method becomes imperative. In this article, besides introducing the respondent driven sampling method, some of the advantages of this method as relative decrease of the bias of estimates, declining the non-response rate by paying incentives and allocating weights proportional to reciprocal of the social network size of respondents to produce unbiased estimates are described. Moreover, some disadvantages of this method such as lack of producing differential samples by selecting similar seeds, lack of reaching more efficient method than snowball sampling by implementing this method improperly and lack of achieving to equilibrium by existing weak social networks among members of interested population are stated. Another aim of this article is to compare sampling methods of hidden population with the respondent driven sampling method which are the results of implementing this method in different surveys and existing simulations.

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Background and Objectives: Changes in ideals and aspirations of childbearing are important factors in fertility behavior. Nowadays, fertility rate reduction below the replacement level and decreased childbearing ideals are the most common fertility challenges in Iran. So, with the decrease in the fertility rate, it is necessary to be aware of the ideal number of children and its determinants in order to adopt suitable population policies contexts. The main objective of this study was to investigate factors affecting the ideal number of children using Poisson regression model.
Methods: In 2012, 389 ever married women aged 15-49 in Semnan Province were selected using two-phase stratified random sampling method and studied through applying a structured questionnaire. To model the ideal number of children by Poisson regression model, marriage duration has taken as offset and the number of children, job status, education level, marriage type, and resident place were considered as predictors. The model was fitted with SPSS software version 22.
Results: All predictors in this study had significant effects on ideal number of children in Semnan (p-value <0.05). Women’s ideal number of children who had 2 or fewer children, were employed, and had university education with consanguineous and rural  marriage was higher than those who had 3 and more children, were unemployed, and had elementary and secondary education with inter-family and urban marriage.
Conclusion: To model the ideal number of children, since it is discrete and count, a Poisson regression model is more efficient as compared to linear regression model.

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Background and Objectives: One of the most important determinants of the fertility level is the birth interval. Considering the importance of this issue, the aim of this study was to analyze the first and second birth intervals using shared frailty survival model and comparing factors affecting these intervals.
 
Methods: Probability proportional to size stratified sampling was used to select 610 married women aged 15-49 years from different regions of Tehran during the winter and spring of 2017. Data were collected using a structured questionnaire. The shared frailty survival model was fitted to investigate the effect of calendar period, age at marriage, education level, education level of the spouse, job and migration status, household costs, and place of residence on first and second birth intervals.
 
Results: The median length of the first and second birth interval was 38 and 55 months, respectively. Calendar period had a significant effect on the first birth interval (p-value=0.016). Job status (p-value=0.045) and place of residence (p-value=0.025) had a significant effect on the second birth interval. The hazard rate  of the first birth interval for women in the recent calendar period compared to women in the first period was equal to 0.448, and the hazard rate of the second birth interval for employed compared to unemployed, living in developed versus undeveloped regions was 0.812, and 0.724, respectively
 
Conclusion: Delayed childbearing among young women and longer second birth intervals in employed women may result from economic and social conditions that can be prevented by providing appropriate conditions.

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