Study design, population and study sites
This is a cross-sectional study conducted to determine the SRH needs of SIYPs in two states in southwestern Nigeria. It was a follow-up to the water access, sanitation and hygiene community engagement program reported by Folayan et al . The two States – Lagos and Osun - were purposively selected based on their level of industrialization. Lagos is an industrialized cosmopolitan State while Osun is a less industrialized State.
The study population were male and female adolescents aged 10 to 24 years, living ‘on’ and ‘of’ the street. At each study location, places where SIYPs aggregate in large numbers such as major streets, market places and motor-parks were identified through a mapping exercise conducted by the research team along with officials of the State Ministry of Health. These locations were grouped as clusters. The clusters in Lagos State were Bariga and Ajah while that for Osun State were Oke-Baale, Olaiya and Sabo.
The determination of sample size for this study was guided by Turner  recommended estimates to derive sample size for surveys on orphaned and vulnerable children (OVC) in homeless situations. Considering the unavailability of data to generate a prevalence rate of OVC in the proposed study environment, the suggested minimum sample size of 800 to 1000 was adopted and 1,505 street-involved young people were enrolled in this study.
Study recruitment procedure: Following community engagement, the study participants were recruited using respondent-driven sampling and time-location sampling methods [23, 24]. The respondent-driven sampling developed by Heckathorn  is a sampling process whereby participants recruit their peers, in hard-to-reach study populations . In this study, the first ten seeds recruited through the respondent-driven sampling were given a labelled tag with generated identity numbers. After their enrollment, they were given two additional tags to recruit friends/peers. Each referred respondent was checked for eligibility, enrolled and interviewed once the eligibility criteria were met. The eligibility criteria were: aged 10–24 years, living ‘on’ or ‘of’ the street in Lagos or Osun State and mentally stable.
Study participant recruitment through respondent-driven sampling was slow, as there were identified boundaries within street groups and low density of social networks. The research team then adopted time-location sampling to recruit the target population at specific times, days and venues where SIYP gather . To limit selection bias with this method, we ensured that the venue-day-time options with possible large turnouts of SIYP were selected for the recruitment of participants.
Through the respondent-driven sampling, 34 seeds were recruited, and 465 coupons were given out over six recruitment waves within five days. Only 120 respondents were recruited and interviewed through this method. In contrast, the time-location sampling method resulted in 1,800 coupons given, and 1,385 respondents recruited and interviewed [See Supplementary File 1 for study recruitment characteristics].
The questionnaire was administered in a place the respondent identified to be most comfortable to respond to questions. Data were collected by the field worker electronically using REDcap - a secure web application used to build and manage online surveys/databases.
The research instrument was adapted from the “Illustrative questionnaire for interview-surveys with young people” designed by John Cleland for the World Health Organisation . The content of the questionnaire was revised by two experts in sexual and reproductive health, to address the study objectives and fit within the Nigerian cultural context. The revised questionnaire was pre-tested with 20 purposively selected SIYPs at two locations aside the study sites (Ile-Ife and Ibadan), to ascertain the clarity and conciseness of the questions. The tool was revised for language and procedural clarity, then translated into the local dialect (Yoruba) for respondents that do not understand English language. Field workers that speak the other national languages - Ibo and Hausa – were made available to interpret key concepts to study participants where necessary.
Measurement of variables
The explanatory variable in this study was ‘knowledge of SRH’ measured by (i) knowledge of modern contraceptives; and (ii) access to SRH information. A knowledge of modern contraceptives was deduced from responses to a question asked from respondent - “do you know of any of these methods which men and women can use to prevent pregnancy?” There were seven options – (i) injection, (ii) condom, (iii) emergency contraception, (iv) traditional method, (v) withdrawal method, (vi) safe period and (vii) periodic abstinence. Options i-iii were recoded as ‘1’ implying knowledge of modern method of contraception; and others were recoded as ‘0’.
Further, respondents were asked ‘if they ever attended/or were given a talk on SRH’; a positive response was assigned “1”, and otherwise “0”. The respondents were also asked to state the contraceptive method used at last sexual activity by self or partner.
The variables adjusted for in this study were selected background characteristics of respondents: age [10–14, 15–19, 20-24years], educational status [none, primary, secondary] and employment status [not working, working]. The outcome variable was ‘sexual risk behavior’ proxied by three variables: (i) inconsistent use of condom, (ii) multiple sexual partners; and (iii) transactional sex. Both male and female respondents who were sexually active were asked if they used condom at the last sexual activity. An answer in the affirmative was assigned “1”and a negative response was assigned “0”. Respondents were also asked the number of sex partners they currently had. Those who responded they had one were assigned “1” while those with more than one sex partner were assigned “0”. Also, two questions were asked to probe for transactional sex: “have you ever paid or exchange gift for sex?”, and “have you ever been paid or receive gift in exchange for sex?”. An answer in the affirmative to either of the questions was assigned “1” and a negative response was assigned “0”.
Data analysis was conducted using Stata SE 15.1 (Stata Corporation, College Station, Texas). The univariate analysis was conducted to determine the percentage distribution of participants by age in grouped years [10–14, 15–19, 20–24], sex, educational level, employment status, knowledge of modern contraception and access to SRH information. Bivariate analysis was conducted to test associations between the explanatory and outcome variables using Pearson chi-square test. The inferential analysis was conducted using logistic regression to determine the risk indicators for the outcome variables by sex. Two models guided the regression analysis. The first model regressed each of the outcomes against the explanatory variables, while the second model adjusted for confounders (age, literacy level, and work status).
Based on the Pearson chi-square significant association test, we set the p-value cut-off point of 0.20 (p ≤ 0.20) for the inclusion of confounders in the regression model. The Hosmer-Lemeshow goodness-of-fit test was conducted to ascertain all study variables fulfilled the underlying assumption of a univariate regression. Statistical significance was considered at p-value less or equal to 0.05.