Carbon Lighthouse · Data Scientist
Updated · 2026-09-22

Carbon Lighthouse Data Scientist
Interview Questions & Guide 2026

THE 60-SECOND BRIEF

The role of a Data Scientist at Carbon Lighthouse is pivotal in leveraging data to drive sustainability and energy efficiency solutions. As a Data Scientist, you will be tasked with analyzing complex datasets to develop actionable insights that influence strategic decisions, enhance product offerings, and ultimately contribute to the company's mission of reducing carbon emissions. You will engage with a variety of data sources, employing statistical methods and machine learning techniques to uncover trends and optimize processes.

Most of the loop measures decision-making under uncertainty rather than recall. You are scored on whether you state your assumptions, commit to an estimate you can defend, and say explicitly what evidence would change it.

Carbon Lighthouse candidates report 2 rounds · ≈ 2-4 weeks. The stages below are what candidates describe, not a published process.

Set fraud thresholds by expected costRead vintage curves, not blended portfolio averagesReport only matured cohorts for loss metrics

27 min read

Browse Data Scientist questions

See the practice prompts

14Practice promptsAcross five skill areas
3With worked solutionsIncluded in the practice prompts

The role of a Data Scientist at Carbon Lighthouse is pivotal in leveraging data to drive sustainability and energy efficiency solutions. As a Data Scientist, you will be tasked with analyzing complex datasets to develop actionable insights that influence strategic decisions, enhance product offerings, and ultimately contribute to the company's mission of reducing carbon emissions. You will engage with a variety of data sources, employing statistical methods and machine learning techniques to uncover trends and optimize processes.

In this role, you will have the opportunity to work on projects that directly impact energy efficiency programs, contributing to innovative products and services that help commercial buildings operate more sustainably. Your work will enable Carbon Lighthouse to enhance its analytical capabilities, improve operational efficiencies, and deliver significant value to its clients. This position not only demands technical proficiency but also requires a strong understanding of the energy sector and an ability to communicate findings effectively to diverse stakeholders.

01

HR Screening

reported

A screening call is a matching exercise run by someone who will not evaluate your statistics. They are checking that the work described on your resume is work you personally did, and that its scope matches the level the role is written for. Logistics get settled in the same half hour so nobody spends an interviewer's afternoon on a mismatch. The answer that fails is the one narrated in the plural. If every sentence is 'we built' and 'the team decided', there is nothing specific to write down about you. Name the piece that was yours, the decision you made inside it, and what changed after.

What to demonstrate

  • Whether the ownership implied by your resume survives one round of follow-up about who actually did which part
  • Whether your described scope (data size, stakeholders, what shipped) matches the seniority the role is written at
  • Whether timeline, location and compensation expectations make the rest of the loop worth scheduling

How to prepare

  • Rewrite your top three resume bullets in the first person singular, each with the decision you made and what moved afterwards, then say them out loud once so the 'we' does not return under pressure
  • Attach one number to each project: the baseline, the change, and the window it was measured over. Where impact was never measured, say that plainly rather than inventing a figure
  • Settle your compensation range before the call and give it as a range with a reason behind it, such as current total comp or a competing timeline, instead of deflecting the question twice
PracHub interview research ↗
02

Technical Interviews

reported

Much of what gets scored here happens out loud while you type. Nobody can see your reasoning inside a half-written query, so five silent minutes read as being stuck even when they are not. State the plan in plain language first: which tables, what grain you are aggregating to, and the one filter that defines the population. Then write it. The narration doubles as insurance, because a wrong plan gets caught early and cheaply while a wrong query gets caught at the end with no time left to redo it. A timed statistics section, where one exists, is a separate test with its own clock.

What to demonstrate

  • Whether the query you write matches the plan you just described
  • What you do with a hint, meaning whether the correction gets absorbed or the first approach gets defended
  • Whether you can debug your own wrong output by reading the result set and naming which part of the query produced the anomaly

How to prepare

  • Solve three problems while screen-sharing into a recording, then watch it back and mark every stretch longer than thirty seconds where you said nothing
  • Practise compressing the plan into one sentence before typing, then check afterwards whether the finished query actually matched it
  • Time yourself on statistics questions that carry a business reading, such as what a confidence interval does and does not claim, rather than re-reading notes without a clock
PracHub interview research ↗

PracHub editorial advice for the preparation topics above.

01

Averaging delinquency across a book that is growing

A loan three months old cannot be 90 days past due, so a portfolio with many recent originations reports a low blended 90+ rate purely from age mix. The blended rate falls fastest exactly when originations grow fastest, which is precisely when credit quality most needs watching, so the metric moves in the reassuring direction during the riskiest period. Only comparisons at equal months on book are valid, which is what a vintage or roll-rate view enforces.

02

Counting authorizations instead of weighting them, and summing amounts across currencies

Declines skew toward high-value, cross-border and card-not-present transactions, so an unweighted approval rate can sit flat while approved value falls. Merchant retry logic also turns one declined purchase into several rows, inflating the denominator by an amount that varies by merchant and by decline reason. Amounts are held in the minor unit of the transaction currency and that unit is not always two decimals, since some currencies have none and some have three, so summing amount_minor across currencies produces a figure with no interpretation at all.

03

Ending an analysis without a recommendation or next step

Close with what you would do and what would change your mind, stated as a condition you can check later. If the evidence is genuinely inconclusive, recommend the specific next measurement and say what it costs in time or exposure.

04

Optimising accuracy on a heavily imbalanced target

State the base rate first, then choose the metric from the relative cost of a false positive against a false negative: precision and recall at the operating threshold, PR-AUC, or expected cost. At a 1 percent positive rate, predicting the majority class for everyone scores 99 percent accuracy and is worthless.

Choose a category, try a prompt, then open its approach, worked solution or follow-up when you need it.

11 technical prompts3 include a worked solution

Describe how you would implement a machine learning model for predicti…

medium
machine learning and modelling

Describe how you would implement a machine learning model for predicting energy usage.

Approach
  1. Pick an evaluation metric that matches the cost of each error type, not a default.
  2. Frame the prediction: the label, the moment of prediction, and the action it triggers.
  3. Say how the offline result would be validated online before it is trusted.
Follow-up
  • Where could label leakage enter this setup?
  • How would you choose the decision threshold, and who owns that choice?

Build a vintage delinquency table without pivot or unstack

easyWorked solution
vintage analysiscohortsgroupbycumulative max

fct_loan_performance_monthly gives loan_id, origination_month, months_on_book, days_past_due, charge_off_flag and restructured_flag. Produce a DataFrame with one row per origination_month and columns for months_on_book 0 through 12, each cell holding the share of that vintage's funded loans that had ever reached 90 or more days past due, or charge-off, by that age. You may not use pivot, pivot_table, crosstab or unstack. Cells for ages a cohort has not yet reached must be NaN rather than zero.

Approach
  1. Define the per-row indicator as days_past_due >= 90 or charge_off_flag, then take a cumulative maximum of it per loan ordered by months_on_book, because the metric is reached-by-age-m, not in-that-state-at-age-m.
  2. Deal with restructuring before the cumulative max. Restructuring resets days_past_due, so a restructured loan re-enters at current and, without the cumulative maximum carrying its pre-restructure worst state, reads as a cure.
  3. Fix the denominator once as the count of distinct loan_id per origination_month across the whole cohort. Prepaid and charged-off loans stop producing rows, so a denominator recomputed at each age silently shrinks exactly where losses land.
  4. Aggregate with groupby(['origination_month','months_on_book'])['ever_90'].sum(), then pre-build the output frame indexed by sorted origination months with integer columns 0 to 12 and assign from the grouped Series by .loc on its index.
  5. Mask cells beyond each cohort's maximum observed months_on_book so an immature cell reads NaN instead of an artificially low rate.
Worked solution 30 min
  1. Sort by loan_id and months_on_book, build the ever_90 indicator, then apply groupby('loan_id')['ever_90'].cummax().
  2. Compute cohort_size as the distinct loan_id count per origination_month, before any filtering on age.
  3. Group the cumulative indicator by origination_month and months_on_book and sum it to get the numerator per cell.
  4. Create the output frame with the sorted origination months as index and range(0, 13) as columns, fill it from the grouped Series, and divide each row by its cohort_size.
  5. Compute each cohort's maximum observed months_on_book and set every cell to the right of it to NaN.
EXPECTED RESULTA DataFrame with one row per origination_month and columns 0 to 12, values non-decreasing left to right within every row, near zero in column 0, and NaN forming an upper-right triangle where cohorts have not yet aged.
Follow-up
  • Two adjacent vintages diverge at months_on_book 6. How would you separate seasoning, mix shift and a genuine credit-quality change?
  • The three most recent vintages look best on this table. What do you check before saying so?
  • How does the table change if charge-off policy moved from 180 to 120 days past due partway through the series?

Bootstrap a fraud loss rate that clusters within merchant

medium
bootstrapclustered resamplingheavy tails

You have a per-transaction frame with auth_id, merchant_id, settled_amount_reporting and net_loss_reporting, both already in one reporting currency. Most rows carry zero loss, a few carry large ones, and losses cluster within merchant. Using only numpy's random generator and no resampling helper from any library, write a bootstrap that returns a 95 percent interval for net fraud loss in basis points of settled volume, resampling merchants with replacement and taking all rows belonging to each drawn merchant. Also produce the naive row-level interval and state which you would report.

Approach
  1. State the estimator before writing it: total net loss divided by total settled volume, times 10,000. It is a ratio of sums, so each replicate recomputes both sums. Averaging per-transaction loss rates instead would weight a five-unit transaction like a five-thousand-unit one.
  2. Pre-aggregate loss and volume to merchant level once. For a ratio of sums, drawing merchants and taking all their rows is arithmetically identical to drawing merchant-level (loss_sum, volume_sum) pairs, so a replicate becomes one integer draw plus two vectorised sums rather than a groupby inside the loop.
  3. Draw B replicates of M merchant indices with replacement, where M is the observed merchant count, compute the ratio per replicate, and take the 2.5th and 97.5th percentiles. Say explicitly that this is a percentile interval and that BCa would correct the skew-induced bias if the decision is close.
  4. Repeat with independent row draws for the naive interval and compare widths on the same replicate count.
  5. Report the clustered interval. Rows within a merchant share an acceptance profile, a category code and a fraud exposure, so they are not independent, and the row-level interval understates variance by roughly the design effect.
Follow-up
  • Your clustered interval is three times wider. How do you explain that to someone who wanted a tighter number?
  • One merchant accounts for 40 percent of losses. What does that do to the interval, and what would you do about it?
  • How does this change if the question is whether two months differ rather than what this month's rate is?

For someone who can already write the query and train the model but stalls when asked what to measure or whether a change is worth making. Metric definition and case structure come first; the technical work is kept as maintenance rather than the centre of the week.

Small steps. Visible outcomes.0 / 7 completed
ONE WEEK · YOUR PACE

Prepare, practise & reflect

One practical outcome each day. Spend longer where you need it.

0 / 7 done
01Metric anatomy
  • For three products you use daily, write one primary metric, two input metrics that plausibly move it, and one guardrail that would catch a cheap way of moving the primary at the cost of the product.
  • For one of them, specify the metric precisely enough that two analysts would return the same number: numerator, denominator, unit of observation, time window, and how returning and deleted accounts are treated.
  • Pick a ratio metric and write what happens to it when the denominator shrinks for reasons unrelated to the numerator, with a concrete example of that happening.

Deliverable: A one-page metric tree for one product, with the primary metric written as an unambiguous spec.

Practice prompt ↗Practice prompt ↗Worked solution ↗
02Diagnosing a drop without guessing
  • Take the prompt "weekly active users fell 8 percent week over week" and write the segmentation plan before proposing any cause: platform, region, tenure cohort, acquisition channel, and whether the movement sits in the numerator or in a changed denominator.
  • List the instrumentation failures that manufacture fake drops (a client release that stopped firing an event, a bot filter change, a shifted date boundary or timezone) and write the query that rules out each one.
  • Rehearse stating the boring explanations first, seasonality and day-of-week composition, before reaching for a product cause.

Deliverable: A drop-diagnosis checklist short enough to recite from memory in under a minute.

Practice prompt ↗Practice prompt ↗
03Should we build it
  • Take a feature idea and write it as a bet: what you believe is true, what would have to be true for it to pay off, the metric that would confirm it, and the effect size that would justify the engineering cost.
  • Size the opportunity top-down and bottom-up, then reconcile the two numbers in writing instead of quoting whichever is friendlier.
  • Write the counter-metric that would make you kill the feature even if it wins on the primary metric.

Deliverable: A one-page product memo ending in a decision rather than a list of considerations.

Practice prompt ↗Practice prompt ↗
04The places aggregate numbers lie
  • Construct a Simpson's paradox numerically: two segments where the treatment wins within each segment yet loses overall, and identify the shift in segment weights that causes it.
  • Take a heavy right-tailed quantity such as revenue per user and write why the mean is the wrong summary, which percentile you would report instead, and what a moving mean with a stable median tells you.
  • Write your definition of a session for the product from day one, then name two real behaviours it misclassifies.

Deliverable: One page holding a worked Simpson's paradox table and a session definition with its two known failure cases.

Practice prompt ↗Practice prompt ↗Worked solution ↗
05Technical maintenance, aimed at metrics
  • Solve four timed SQL prompts that all end in a ratio metric, so the question of grain stays live in every answer.
  • Compute a 95 percent confidence interval for a proportion on a small sample, and state why the normal approximation is unreliable when either np or n(1 minus p) falls below roughly 10, along with which interval you would use instead.
  • Take one metric from your day-one tree, write the query that computes it correctly, then write the query that computes it wrong in the most plausible way and explain how you would notice.

Deliverable: Four solved prompts plus a matched correct and plausible-wrong query for one metric.

Practice prompt ↗Practice prompt ↗
06Turning engineering work into data science stories
  • Write three project stories as situation, decision, trade-off, outcome, each carrying one number and one thing you got wrong.
  • For the story you will lead with, prepare an answer to "what would you do differently" that names a decision you made, not a constraint you were handed.
  • Practise the sentence that reframes a systems project as a question project: the question the work answered, ahead of the pipeline it shipped.

Deliverable: Three written stories with the lead story delivered aloud and timed under four minutes.

Practice prompt ↗Practice prompt ↗
07Mock case and gap list
  • Run a 40-minute mock case with someone playing a product manager who pushes back on your metric choice, and record it.
  • Listen back and mark every moment you proposed a solution before the success metric existed.
  • Rewrite those moments as the question you should have asked, and rehearse the first 90 seconds of the case until scoping comes before solving.

Deliverable: A recorded case plus a rewritten opening 90 seconds.

Practice prompt ↗Practice prompt ↗Worked solution ↗

Expand any day for tasks and deliverables. Your progress is saved on this device.

An answer without a quantity is hard to interrogate, so interviewers keep probing until they find one. Come with the baseline, the change, the window it was measured over, and how confident you were. If the effect never got measured, say so and say what you would have measured. Fabricated precision is worse than an honest gap.

What motivates you to work in the field of data science and energy eff…

medium
behavioural and stakeholder questions

What motivates you to work in the field of data science and energy efficiency?

Approach
  1. State the situation in two sentences and spend the rest on your reasoning.
  2. Pick a story where you drove the decision, not one where you observed it.
  3. Close with what you would do differently, concretely.
Follow-up
  • What would you do differently if you ran that project again?
  • How did you know the outcome was caused by your change?

How do you handle conflicts within your team?

medium
behavioural and stakeholder questions

How do you handle conflicts within your team?

Approach
  1. Quantify the outcome, including what you would not claim credit for.
  2. State the situation in two sentences and spend the rest on your reasoning.
  3. Pick a story where you drove the decision, not one where you observed it.
Follow-up
  • What would you do differently if you ran that project again?
  • How did you know the outcome was caused by your change?

Can you provide an example of how you have contributed to fostering an…

medium
behavioural and stakeholder questions

Can you provide an example of how you have contributed to fostering an inclusive work environment?

Approach
  1. Quantify the outcome, including what you would not claim credit for.
  2. State the situation in two sentences and spend the rest on your reasoning.
  3. Pick a story where you drove the decision, not one where you observed it.
Follow-up
  • What would you do differently if you ran that project again?
  • How did you know the outcome was caused by your change?
  • 01

    What motivates you to work in the field of data science and energy efficiency?

  • 02

    How do you handle conflicts within your team?

  • 03

    Can you provide an example of how you have contributed to fostering an inclusive work environment?

PracHub interview preparation framework ↗
Is this an official Carbon Lighthouse interview guide?

No. It is PracHub's own research and practice material for the Data Scientist role at Carbon Lighthouse. Rounds and questions reflect what candidates have reported, not a process Carbon Lighthouse has published, and they change over time. Confirm the current format and scope with your recruiter.

PracHub interview research ↗
What is the typical interview difficulty for this role?

The interview process is moderately challenging, focusing on both technical skills and cultural fit. Candidates should prepare for a mix of behavioral and technical questions, reflecting the complexities of the role.

PracHub interview research ↗
How long does the interview process typically take?

From initial screening to the final interview, candidates can expect the process to last approximately 2-4 weeks, depending on scheduling and availability.

PracHub interview research ↗
What differentiates successful candidates at Carbon Lighthouse?

Successful candidates demonstrate a strong alignment with the company's mission, a collaborative mindset, and a robust technical skill set. They are able to articulate their thought processes clearly and adapt to different team dynamics.

PracHub interview research ↗
What is the work culture like at Carbon Lighthouse?

The culture is built on collaboration, inclusivity, and a shared commitment to sustainability. Employees are encouraged to bring diverse perspectives to the table and contribute to a positive work environment.

PracHub interview research ↗
Sources & methodology 3 sources ↗

No official company page is cited. Rounds and questions come from candidate reports and PracHub editorial material; each source shows the date it was read.