Revenue recognition  The Company applies recognition of revenue over time, which is similar to the method the Company applied under previous guidance (i.e., percentage of completion). The Company determines, at contract inception, whether it will transfer control of a promised good or service over time or at a point in timeregardless of the length of contract or other factors. The recognition of revenue aligns with the timing of when promised goods or services are transferred to customers in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those goods or services. To achieve this core principle, the Company applies the following five steps in accordance with its revenue policy: ( 1 ) Identify the contract with a customer ( 2 ) Identify the performance obligations in the contract ( 3 ) Determine the transaction price ( 4 ) Allocate the transaction price to performance obligations in the contract ( 5 ) Recognize revenue as performance obligations are satisfied Due to uncertainties inherent in the estimation process, it is possible that estimates of costs to complete a performance obligation will be revised in the near-term. For those performance obligations for which revenue is recognized using a cost-to-cost input method, changes in total estimated costs, and related progress toward complete satisfaction of the performance obligation, are recognized on a cumulative catch-up basis in the period in which the revisions to the estimates are made. When the current estimate of total costs for a performance obligation indicate a loss, a provision for the entire estimated loss on the unsatisfied performance obligation is made in the period in which the loss becomes evident. Disaggregation of Revenues The Companys revenues are principally derived from construction and engineering contracts related to Modules. Our contracts are with many different customers in numerous industries. The foll owing tables provide furt her disaggregation of the Companys revenues by categories: Three Months Ended March 31, Revenue by Customer Type 2019 2018 Multi-Family $ 70,873 4 % $   % Office 1,125,603 65 % 363,629 24 % Retail 530,659 31 % 114,944 7 % School   % 759,835 49 % Special Use 6,812  % 305,118 20 % Other 1,177  %   % Total revenue by customer type $ 1,735,124 100 % $ 1,543,526 100 % Contract Assets and Contract Liabilities Accounts receivable are recognized in the period when the Companys right to consideration is unconditional. Accounts receivable are recognized net of an allowance for doubtful accounts. A considerable amount of judgment is required in assessing the likelihood of realization of receivables. The timing of revenue recognition may differ from the timing of invoicing to customers. Contract assets include unbilled amounts from long-term construction services when revenue recognized under the cost-to-cost measure of progress exceeds the amounts invoiced to customers, as the amounts cannot be billed under the terms of our contracts. Such amounts are recoverable from customers based upon various measures of performance, including achievement of certain milestones, completion of specified units or completion of a contract. Contract assets are generally classified as current within the condensed consolidated balance sheets and labeled as costs and estimated earnings in excess of billings on uncompleted contracts. Contract liabilities from construction and engineering contracts occur when amounts invoiced to customers exceed revenues recognized under the cost-to-cost measure of progress. Contract liabilities additionally include advanced payments from customers on certain contracts. Contract liabilities decrease as the Company recognizes revenue from the satisfaction of the related performance obligation. Contract liabilities are generally classified as current within the condensed consolidated balance sheet and labeled as billings in excess of costs and estimated earnings on uncompleted contracts. Although the Company believes it has established adequate procedures for estimating costs to complete on open contracts, it is at least reasonably possible that additional significant costs could occur on contracts prior to completion. The Company periodically evaluates and revises its estimates and makes adjustments when they are considered necessary. Cash and cash equivalents  The Company considers cash and cash equivalents to include all short-term, highly liquid investments that are readily convertible to known amounts of cash and have original maturities of three months or less upon acquisition. Cash and cash equivalents totaled $250,464 as of March 31, 2019 and $ 1,368,395 as of December 31, 2018 . Short-term investment  The Company classifies any investment with a maturity greater than three months but less than one year as short-term investment. The Company had no short-term investments as of March 31, 2019 or December 31, 2018 . Accounts receivable and allowance for doubtful accounts  Accounts receivable are receivables generated from sales to customers and progress billings on performance type contracts. Amounts included in accounts receivable are deemed to be collectible within the Companys operating cycle. The Company recognizes account receivable at invoiced amounts. The allowance for doubtful accounts reflects the Company's best estimate of probable losses inherent in the accounts receivable balances. Management provides an allowance for doubtful accounts based on the Companys historical losses, specific customer circumstances, and general economic conditions. Periodically, management reviews accounts receivable and adjusts the allowance based on current circumstances and charges off uncollectible receivables when all attempts to collect have been exhausted and the prospects for recovery are remote. Recoveries are recognized when they are received. Actual collection losses may differ from the Companys estimates and could be material to its consolidated financial position, result of operations, and cash flows.