The adoption of Public-Private Partnership (PPP) in procuring construction projects started in Nigeria in the early 90s which allows the private sector to be involved in the construction of mass housing [ 1,2,3]. Due to the housing crisis across Nigeria, The Federal government of Nigeria was compelled to engage in mass housing construction basically to meet the housing needs of the citizenry. However, Oluikpe [ 2] and Akpan [ 4] stated that the various policies regarding the housing schemes of the Federal Government at providing mass accommodation at an affordable cost for low and middle-income earners have failed woefully, hence the adoption of PPP in the sector. Abuja; the federal capital territory was chosen as the pioneer city for the PPP mass housing scheme in the year 2000 to be superintended by the Federal Capital Development Authority [ 5]. Muhammad and Johar [ 6] also opine that Abuja city has the largest proportion of PPP housing projects and a conglomeration of experts in the industry, hence the need for the study area. It was also observed that PPP-MHP is not performing to cost, time, and quality since the scheme is also experiencing cost and time overrun that is common in the conventional method of procurement as a result of underlying risks inherent in PPP procurement [ 7]. It is against this backdrop that this study becomes imperative with a view to assessing the degree of occurrence and severity of risk factors in PPP-procured mass housing project delivery in Abuja, Nigeria.

Housing is a complex product that is crucial for national development in terms of both economy and welfare. Uwatt [ 8] submits that the housing sector plays a very important role in the social and economic development of Nigeria due to its impact on macroeconomic indicators such as employment, savings, investment, and labor productivity. Investment in housing at such a significant level stimulates the demand for labour in the construction and building materials industries and thus affects income production in the economy. Okwu, Ngoepe-Ntsoane [ 9] opined that housing promotes investment opportunities that are capable of promoting sustainable growth and the development of an economy. Therefore, adequate provision of decent housing is an important measure of social welfare and economic development in any nation which also serves as an international human right. Festus and Amos [ 10] believed that the Nigerian housing market has tremendous opportunities that are waiting to be tapped since Government alone cannot fill the housing gap. According to UN Habitat [ 11], the housing sector has strong linkages for stimulating economic growth and development most especially in a depressed or stagnant economy like Nigeria’s recent case. Investment in the sector will raise the standard of living of the people. However, Harris and Arku [ 12] opined that governments and academics were slow to recognize the potential of housing as a sectorial tool of economic development. This is a direct result of the dearth of affordable and decent housing for low and medium-income earners in Nigeria hence the adoption of PPP in the sector.

The PPP MHPs in Abuja, Nigeria are superintended by the Federal Capital Development Authority (FCDA). It is collaboration between the government and the Organized Private Sector (OPS) irrespective of the PPP options adopted. Ahadzie, Proverbs [ 13] opined that the MHPs represent the largest project-based sectors of the Nigerian construction industry. The MHPs are described as the mass production of housing development in a particular region or area usually owned by the public [ 14]. The features and characteristics associated with MHPs include the same project environment; same site conditions including topography; weather; bulky materials and design considerations. PPP- MHP as described in this study includes the design and construction of speculative standardized house units usually in the same location and executed within the same project scheme. Such house units could include a terrace, multi-story or tower blocks, maisonettes, semi-detached and/or detached residences, or a combination of them. Four main points are worthy of note as regards the description viz. (a) The PPP-MHPs are based on one or more standardized designs in the sense that the architectural design of all the phases is the same for all house units. This is necessary to ensure that the concept of repetition is met. (b) The PPP-MHPs involve the construction of domestic residences. (c) The PPP-MHPs are speculative in the sense that the acquisition of land, design of house units, and construction are made without reference to any specific customer in mind and (d) the PPP-MHPs are located in the same area and be part of the same scheme and/or contract conditions.

Risk has been described with a different word and interpreted with a different meaning in the construction management literature [ 15]. Baloi and Price [ 16] defined risk as exposure to the chance of occurrences of events that either adversely or favourably affects project objectives as a consequence of uncertainty. This definition points to the inherent characteristics of risk like risk event(s)/factor(s); likelihood of occurrence; consequences (losses/gain) and uncertainty [ 17]. Nicholas and Steyn [ 18] stated that the probability of occurrence of risk is considered together with the severity or consequence of a risk which implied that risk is a joint function of likelihood and severity. The likelihood measured is the probability or the chance of some risk event occurring while severity represents the extent of damage or loss if the risk factor eventuates. Winch [ 19] defines risk as the condition where information is missing in which a probability distribution can be assigned to the occurrence of events about which a decision needs to be taken. This can only be done if a reliable and appropriate data set is available from which the probability distribution can be calculated i.e. if there is adequate knowledge about the past, then probabilities can be used to increase the predictability of future events. Odeyinka [ 20] cited Winch (2010) to clarify the distinction between uncertainty and risk by adopting a cognitive approach to partition risk space into four compartments. The cognitive approach makes a clear distinction between when a probability distribution can be assigned to the occurrence of an event and the condition where it is not possible to assign a probability distribution due to the amount of information available per time. The multilayer definition of risk and uncertainty by Winch (2010) is as follows:

Known-known are the cognitive condition of risk, where the risk source has been identified and a probability can be assigned to the occurrence of a risk event given the risk source.

Known unknowns are the cognitive condition of uncertainty where a risk source has been identified but a probability cannot be assigned to the occurrence of the risk event.

Unknown known is the cognitive condition of uncertainty where somebody knows about the risk source and associated probabilities but is keeping that information private.

Unknown unknowns are the cognitive condition of uncertainty where the risk source has not been identified and therefore the risk event cannot be known

As stated earlier some researchers have tried to distinguish between risk and uncertainty but as observed by Ameyaw [ 17] it will be difficult to distinguish between risk and uncertainty in real PPP projects; take for instance, the OPS in a PPP MHPs scheme may be uncertain whether the government will expropriate investment or renege on its obligation in the future and it is common to find both the case of risk and uncertainty in PPP MHPs. Literature has shown that risk in PPP projects is classified as site acquisition risk, design risk, construction risk, demand risk, political risk, operation risk, financial risk, and force majeure risk. Sanda, Anigbogu [ 15] submitted 74 risk factors associated with PPP housing projects in Nigeria. Awodele [ 21] identified 68 risk factors in PPP market-related projects and classified these risk factors into endogenous and exogenous risks. Awodele [ 22] identified 46 risk factors involved in housing projects procured using the public-private partnership system in Nigeria. This study adopted 63 risk factors from an earlier Delphi study conducted on PPP-MHPs [ 5].

A questionnaire survey was conducted on the level of risk occurrence and its severities on PPP-procured mass housing projects in Abuja, Nigeria. A total of 328 questionnaires were distributed out of 560 identified populations to various categories of respondents in the study area for six months while 276 of the distributed questionnaires were returned and 258 were fit for further analysis representing a 78.66% response rate. This response rate is above the usual rate of 20-30% for questionnaire surveys in construction management studies Moser and Kalton [ 23]. Mean item score (MIS), standard deviation, and Kruskal Wallis analytical techniques were adopted for the study. To determine the level of risk occurrence and severity in the PPP-MHPs sector, the respondents were asked to score on a Likert scale of 7 where scale 1 represents extremely low occurrence/severity; scale 2 = Very low; scale 3 = Low; scale 4 = Moderate; scale 5 = High; scale 6 = Very high and scale 7 represent extremely high occurrence/severity. The MIS was determined using the equation; $MS=\frac{7_{n7}+6_{n6}+5_{n5}+4_{n4}+3_{n3}+2_{n2}+1_{n1}}{N}$

Where MS = mean score of the variable; n = score given by respondents based on the 7-point scale from one to seven and N = several respondents that rated the variable while the Kruskal-Wallis test is used to test the differences between several independent groups and this was used to determine the significant difference in the opinions of the respondents on the listed variables.

Table 1 andTable table 2 show the profile of the respondents where 71% of the respondents come from the public sector while the remaining 29% are from the Private sector. 37.2% of the respondents have a first degree, 47.3% of the respondents have 2^nd degree while 2.3% and 13.2% of the respondents have HND and PGD as their highest academic qualifications respectively. Based on PPP experience, 42.2% of the respondents have between 11-15 years of experience, 36.4% have between 6-10 years of experience, and 20.2% have between 16-20 years of experience while the remaining 1.2% have between 1-5 years of experience with an average of 12 years PPP experience with 9 years average of PPP MHP experience. This is evident as experimenting with PPP in the procurement of Mass Housing is still a relatively new experience in the country. Professionally, the majority of the respondents (72.1%) have corporate membership of the various professional institutes, 11.2% have attained the highest honor (Fellow) of their various professional bodies, and 1.2% were still probationer members of their various professional institutes while the remaining 15.5% have professional qualification in other allied professions.

The mean score of respondents from both the Public and Private sectors that engaged in PPP procured Mass Housing projects on the likelihood of occurrence of risk factors and severity are analyzed and presented in Tables 3, 4, 5 and 6. The mean index of risk occurrence in PPP-MHP in Abuja ranges from 4.00 to 6.60 which suggest that the likelihood of risk occurrence ranges from moderate to extremely high levels. The top 10 factors accounted for the availability of finance (M.S, = 6.60), high finance cost (M.S, = 6.27), non-involvement of the host community (M.S, = 6.06), poor execution of housing policies (M.S, = 6.06), corruption and lack of respect for the law (M.S, = 6.06), wrong perception of housing need by low-income earners (M.S, = 5.97), illegal title to land (M.S, = 5.94), land acquisition and site availability (M.S, = 5.91), level of demand for the mass housing projects (M.S, = 5.82) and unstable value of the local currency (M.S, = 5.78). All the risk factors listed recorded a very high level of occurrence; however, the respondents are different significantly on twenty-nine (29) of the sixty-three (63) in terms of occurrence in PPP mass housing projects. Some of the risk factors with a significant difference include high cost of finance, illegal title to land, high bidding cost, poor decision-making process, delay in project approval and permits, land grabbing/encroachment, possible expropriation/nationalization of assets, import and export restrictions, environment, staff crises, and late design changes. The aforementioned risk factors have p-values ranging from = 0.000.

The mean score of respondents was also conducted on the severity of the risk factors in PPP-procured mass housing projects and is presented inTable 3. The mean index of risk severity ranges from 4.14 to 6.54 which suggest that the severity of the risk factors ranges from moderate to extremely high levels. The top 10 factors accounted for the availability of finance (M.S, = 6.54), high finance cost (M.S, = 6.22), corruption and lack of respect for the law (M.S, = 6.03), wrong perception of housing need by low-income earners (M.S, = 5.98), land acquisition and site availability (M.S, = 5.92), illegal title to land (M.S, = 5.90, SD = 0.397), non-involvement of the host community (M.S, = 5.90, SD = 0.567), poor execution of housing policies (M.S, = 5.88), level of demand for the mass housing projects (M.S, = 5.83) and inadequate experience in PPP (M.S, = 5.79). All the risk factors listed recorded a very high level of severity; however, the respondents differ significantly on forty (40) of the sixty-three (63) in terms of risk severity in the PPP mass housing project. Some of the severity risk factors with a significant difference include high cost of finance, illegal title to land, high bidding cost, poor decision-making process, delay in project approval and permits, land grabbing / encroachment, possible expropriation/ nationalization of assets, import and export restrictions, environment, staff crises, and late design changes. The aforementioned risk factors have p-values of 0.000.

It is evident in this study that the 63 risk factors listed for the respondents were found to either be extremely high, very high, high, or moderate in terms of occurrence while all the listed risk factors recorded a very high level in terms of severity; this is to identify the key risks that could significantly influence the delivery of PPP MHPs in Abuja, Nigeria. The top ten (10) risk factors occurring with a high level of severity in PPP MHPs in Abuja are discussed briefly, namely:

Availability of finance or financial risk is the most risk factor occurring in PPP-MHPs in Abuja, Nigeria according to this study with a probability of 6.60 and the highest severity of 6.54. Ameyaw [ 17] submits that the major risk affecting the attraction of private investment is the non-availability of finance owing to the failure of the stakeholders to fulfil investment commitment, most especially in developing counties like Nigeria and this makes private investment to be risky, where investors are unwilling to commit huge financial resources without a government guarantee. The financial effects of these risks lead to lower productivity, poor performance, and an increase in the cost of the project.

Prevailing economic conditions in the operating environment beyond the control of the organized private sector (OPS) could lead to high financing costs for the PPP MHPs. This risk factor has the 2^nd most occurring risk factor in the sector with a probability of 6.27 and severity of 6.22. As good as the PPP initiative is in the sector, a high finance cost is stalling its delivery resulting in delays and poor workmanship [ 24]. A private firm might be inaccurate in submitting her estimates during the bidding stage as a deliberate attempt to win the contract while underestimating the volume to construct and maintain the estate. This activates poor operating cost controls which threaten successful service delivery.

Corruption is a global problem that poses a serious threat to the development of a country and its people. States, developed or developing, are equal victims of this problem [ 25]. Corruption, apart from affecting the public at large, also causes reduced investment, lack of respect for the rule of law and human rights, undemocratic practices, and diversion of funds intended for development and essential services, affecting the government's ability to provide basic services to its citizens. Corruption and lack of respect for the law can be seen as the behaviour of corruption of government officials that will increase the relationship between the government and the project company; it also includes the government's inconsistent application of new regulations and laws.

Empirical evidence indicates that compensation can prove effective in gaining public acceptance for citing facilities in a particular location or community and this can only be effective when the host community is involved before citing such facilities in their neighborhood [ 26]. These facilities require creative mitigation measures such as independent inspections of the facility and local shutdown power. Even then, they may be viewed as too risky to be acceptable with or without compensation. Villagers may felt that the unequal distribution of economic benefits within their village was a considerable challenge to participate in such a programme. Members of the community benefiting from the programme are those who are not the indigene of such a community hence they may be less concerned about the success of such a programme. This might be the case of PPP-MHPs as the scheme is not performing to expectation [ 27].

WHO [ 28] stated that there are numerous examples of implemented policies that support healthy housing globally but there exist gaps in the existence of policy across all countries. Nigeria is perennially plagued by myriad problems which have contrived the nation's drive for socio-economic growth and sustainable development. Consequently, successive governments over the years have battled to alleviate the nation of its socioeconomic, environmental, and geopolitical challenges, thus beleaguered by pervasive inadequate housing [ 29]. This indicates a palpable lack of sustainable housing in the country and calls for the need to institute more strategic plans, programs, and policies to ensure the sustainable development of safe, affordable, and robust housing for the populace. The current state of housing development in the country remains poor despite vast resources expended through various schemes over the years, thus there is a need for a change in the perception of housing policies by stakeholders in the country.

The wrong perception of housing needs by low-income earners might be connected with the inadequate mortgage finance in the country and the low quality of the finished product. The history of housing finance in Nigeria had been a terrible one [ 30], some of these constraints were identified as unstable macroeconomic conditions, a weak legal framework for property rights, lack of mortgage market infrastructure, and unavailability of funds for long term finance intermediation but poor access to finance was the major identified impediment to having affordable housing in Nigeria brought about by the wrong perception [ 31].

Land registration generally describes systems by which matters concerning ownership, possession, or other rights in land can be recorded (usually with a government agency or department) to provide evidence of title, facilitate the transaction, and prevent unlawful disposal which varies according to jurisdiction [ 32]. The basic idea behind title registration is to confer on every owner or purchaser a title guaranteed by the state. The formal land market (the Land Use Act) is unable to meet the demand for urban land when and where needed for the teeming urban population. This is because of its implementation process which is time-consuming, frustrating, cumbersome, and highly bureaucratic [ 33]. Some members of the OPS might have access to more than the allotted land space due to their connections to people in government which consequently encourages land speculation which is not healthy for urban land management.

In most PPP contract arrangements, the host government will provide the means of acquiring land for the Private partners to construct the facility. Most of the time, the government does this through its power of compulsory acquisition also known as expropriation of assets or compulsory purchase. Land availability at the appropriate time and reasonable price are critical to the delivery of PPP Mass housing projects at early completion and at reduced development cost; however, the party that bears the risk varies from project to project owing to the contract clauses and conditions.

A low level of demand or decline in the demand for the completed PPP-procured Mass housing projects by the general public may be a result of high inflation and an increase in interest rate. Makinde [ 34] stated that there is a gap in knowledge between the requirement for housing and the ability to obtain the preferred housing type, which results in an effective request crisis for affordable housing in the country. Although it is clear there is a housing shortfall, it is fundamental to know that people can only obtain what they can to meet the expense.

Adequate PPP experience may be lacking, particularly in a sector where PPP is still emerging like the housing sector. This risk is common across public agencies, lack of PPP experience and knowledge by government representatives indicates a lack of capacity to negotiate contracts and guarantees from multi-national lenders to negotiate successful PPPs. Tati [ 35] stated that the lack of attention or experience to handle risk transfer, competition, and contestability eroded the PPP's capacity among the stakeholders and this leads to strained relationships, protracted negotiations, and project implementation problems.

The study reported on the rate of risk occurrence and severity associated with PPP-procured mass housing projects this is made possible by previously piloting the study through a two-stage Delphi procedure in which the risk associated with the sector was established. Sixty-three risk factors were found to be associated with PPP-procured mass housing in Nigeria and these were subjected to descriptive and inferential statistics. In terms of the likelihood of occurrence of risk factors on PPP MHPs, the top 10 factors with a high probability of occurrence are the availability of finance, high finance cost, non-involvement of the host community, poor execution of housing policies, corruption and lack of respect for the law, wrong perception of housing need by low-income earners, illegal title to land, land acquisition and site availability, level of demand for the mass housing projects and unstable value of the local currency. All the risk factors listed recorded a very high level of occurrence and high level of severity.

The implication for practice is that it provides the stakeholders in PPP-MHPs with comprehensive checklists that can aid in developing PPP risk assessment guidelines in the sector though both partners should be aware of the dynamic nature of risk because new ones might be emerging especially in the PPP MHP sector. The divergent view of the PPP partners involved in MHPs reinforces the short history and lack of PPP experience and expertise in the country. Awodele (2012) opined that the failure of some privatized projects in Nigeria was due to the short history and lack of PPP experience and expertise in the country. The study, therefore, recommends a risk management culture to be cultivated among the PPP Partners in the sector. The sector is expected to first generate a list of potential risk events on a PPP project as it was observed that there are no formalized approaches for allocating risks in PPP Mass Housing projects It was observed that the investment in the sector was highly risky, thus we also recommend that both the Public and the private sectors should endeavor to reduce or minimize the risk level by providing incentives that will encourage financial investment in the sector to bridge the housing need in the country.